IF you take it back to UK each year to have it MOT'd. I'm afraid the ones they issue in Spain are not valid for a UK reg car. Yes, they will do it and take your money, thank you very much, but you can only get an MOT for a UK car at a UK MOT testing station.
The car can stay here if you are a non-res but you can't. If you visit Spain more than 183 days in one year the car must be put on to Spanish plates.
The car must also be road legal in the country of registration, ie tax, insured and MOT up to date.d
If the MOT runs out you would be hard pushed to get it back to UK to get it done without it being put on a car transporter.
I know, I know. Tons of people running around with no tax, Spanish MOT and probably no insurance as well. That doesn't make it legal though.
Most Insurance companies will reduce your cover to 3rd party only for a maximum of 3 months unless you pay extra.
Check with your insurance company. Some say all year cover but stipulate no more than 90 days in any one trip (I think Saga is the only one to give more). Co-op covers you for multi trips up to six months but, again, no more than 90 days in any one trip.

Yes you can insure a UK car here with a local company and lots of people do, without incident. But the point that Jan made right at the start is the over-riding one.
You are legally only allowed to keep a UK registered Car in Spain for 6 months in any 12 month period. The car must be fully road legal, have a UK mot (a real one) and have UK tax for the whole period of your stay.
This means..
You cannot use the last 6 months of one year and the first 6 months of the next and stay for a whole year. The 6 month rule is measured over ANY 12 month period.
Car insurance companies will take your premium no problem, but it is up to you to obey the legal road rules. If you are involved in a serious (or expensive) accident, the insurance company can demand proof that you have not exceeded the 6 month rule, as will the police! You have to prove that the car has not been in the country for more than 6 months.
It doesn't matter at all whether you are resident or not. The legality issue is regarding your car, not you.
But while we're on the point, you might want to read the threads on here regarding the legality of your driving licence if you are out of the UK for more than 6 months.
A Spanish ITV does not replace a UK MOT. The ITV on UK cars is only used to check its conformity in order to transfer it to Spanish plates. It does not make it a legal car on Spanish roads if your MOT has run out.
The police have issues with illegal cars, which yours will be if it is in Spain for over 6 months. A few years ago, they spent a day at the gates of Quesada enforcing this rule and eventually gave up after they filled 3 car transporters with cars which were illegally in the country.
UK MOTs are only valid when taken in the UK. You can "buy" them here as we all know, but they are obviously completely illegal.
So you can ignore all of this and be fine, we all know this, but if you are unlucky enough to have a bad accident, even if it's not your fault, the police will throw the book at you. It's your choice to make.

Trotter wrote:"...You are legally only allowed to keep a UK registered Car in Spain for 6 months in any 12 month period..." Where have you got this information from? I haven't seen anything that stipulates that a non resident cannot leave his/her UK legal car in Spain. However, the non resident cannot stay longer than 183 days but the car's fine as long as it's fully UK road legal.

As far as I know the law says you can only keep a foreign registered car in Spain and drive it in Spain for a maximum of 6 months – after that you have to have it registered onto Spanish plates. Obviously during that time you have to make sure that the car remains fully legal with a UK MOT, road tax and insurance. Apparently if you decide not to put it onto Spanish plates then you have to remove the car from Spain as you are allowed only to drive it in Spain for 6 months in any calendar year on UK plates.

If you Google the question this is always the only way. There doesn't seem to be any other way other than to "take a chance" which is not advised but has been talked about on this thread.

Jan

Trotter is correct for a non resident

Bringing Your Vehicle to Spain

Understand how to import a vehicle to Spain and the rules regarding registering it once you arrive...

It is illegal for residents of Spain to drive a car with foreign plates, so once a car is imported to Spain, it must be registered with the Spanish authorities. If entering Spain with foreign plates, residents are allowed to drive from the port of entry to their residence only. Spanish plates must be obtained within 30 days of entry. Temporary permission to drive an imported vehicle - and temporary licence plates - should be obtained from the provincial Traffic Department (Jefatura Provincial de Tráfico) or full re-registration of the car must be undertaken.

Non-resident EU citizens

EU citizens who are not Spanish residents may drive their foreign-registered vehicles (cars, motorbikes and caravans) in Spain for up to 180 days (six months) in a calendar year as long as it is road-worthy in the country in which it is registered. In this case, only the owner of the vehicle may drive it. Students from another EU country studying in Spain are permitted to temporarily import a car for the duration of their studies, but proof of enrolment in an academic institution will be requested.
A foreign-registered vehicle may remain in Spain for more than 180 days in a calendar year as long as it is not driven. Even though EU citizens are no longer required to obtain a residence card, after 182 days the vehicle must be registered in Spain or garaged.
Note: the six month period need not be consecutive and is counted over the course of a calendar year only. This period is based on the owner's and not the car's presence in Spain.
The procedures relating to importing and registering a foreign vehicle are complex. It is possible to complete the process independently, but there are specialist companies that can provide assistance.
In order to register a car in Spain, a vehicle must undergo a safety inspection similar to the UK's MOT test. Import taxes as well as IVA may be payable on imported vehicles once registered.

Non-resident, non-EU citizens

A car owned by a non-EU citizen may be kept in Spain as long as it is only used for up to 180 days (six months) in a calendar year and remains road-legal in its country of registration. However, once it has been used for six months, it should be officially sealed (precintado) by Customs authorities (aduana) or the Guardia Civil. Though not commonly done, it can be used as proof that the vehicle was not driven illegally. At the start of the next calendar year, the seals may be removed by the officials and the vehicle may be driven again for another 180 days. There is a fee for this service.

Thank you for pointing out that I wasn't specific enough or clear in my initial post Freddo. That's the benefit of these forums.. We can all discuss and clear up any points as required. It's like a "virtual bar" where the worlds problems get discussed! (Does that make you the landlord Jan??)
I wrongly assumed that the original post was referring to having and driving a uk registered car in Spain for an extended period during one stay. I should have realised that it was more specifically about keeping the car in Spain, regardless of the owner being here..
First, Freddo is quite correct.. A UK registered car can be used on Spanish roads for up to 6 months in any calendar year in Spain, as long as the legal requirements are met to keep it road legal. However, if stopped and the Guardia even SUSPECT that you have been using it for more than 6 months in a calendar year, it is up to you to prove otherwise, thus the existence of the ability for the Guardia Civil to come round and "seal" the car when you're not here. This is sufficient proof. But I really would be surprised if anyone did this. I've never known anyone to have it done, but maybe someone else has?
Secondly, if someone keeps a car here for "as long as they like", that must surely be to a maximum of 12 months, minus travelling time back to the uk, in order to get an mot carried out to keep it road legal? Unless of course they never intend to drive it again, or they will have it transported back to the uk to get its MOT, both of which seem unlikely.
i also seem to recall from a previous discussion that the rules were very different not only for those who are resident, but also those who simply own a property in Spain.
If you own a property in Spain (regardless of whether you have residencia or not) then legally the police can apparently fine you for driving after 30-days in a foreign registered car. Perhaps someone knows more about this?

Thank you Freddo for your evidence based input. One other thing worth knowing if you bring your car over for an extended stay. Even when everything else is legal and by the book, if you leave your car in Spain and you fly back to the UK for a few days your car insurance is not valid whilst you are not in Spain with it. I was told this by SAGA so it probably applies to them all. Not too big a risk if you leave it in a secure place whilst you are away but say you leave it at airport parking......We had to fly back for a wedding one year during a 3 month visit to Spain in the UK car. Ignorance is bliss, but now that I know I won't do that again!

I think you have been misinformed by SAGA. If they are correct then most of the cars parked at airports dont have Insurance? I don't think so.

Wong information from SAGA
This is an extract from their policy

You are not covered for any of the following:
1. Use of the vehicle
When your vehicle:
n is being driven by or is in the charge of any person not allowed to do
so under your Certificate of Insurance other than in an emergency as
defined under Part 2 of the ‘Liability to others’ section;
n is being used other than for the purposes shown in your Certificate of
Insurance;
n is being driven with your permission by any person who you know has
never held a driving licence; is disqualified from holding or obtaining a
driving licence; or is breaking the conditions of their driving licence;
n is towing for reward a caravan, trailer or disabled mechanically
propelled vehicle;
n is towing more than one caravan, trailer or disabled mechanically propelled
vehicle at any one time.
This exception does not apply if your vehicle is:
n being serviced or repaired by a member of the motor trade;
n stolen or taken away without your permission;
n being parked by a car-parking service.

Further to my comment about home owners having different rules regarding driving UK registered cars in Spain and to provide the suggested "evidence based input", I managed to find a couple of references to this.. Most notably on the "carregcostablanca" website which is very specific to this topic and states..

"Q: How long can I legally drive my foreign registered car in Spain?
A. Legally you are able to drive a foreign registered car in Spain for 180 days if you are on holiday. However you must have a valid UK MOT, road tax disc and insurance to drive whilst abroad. If you own a property in Spain (regardless of whether you have residencia or not) then legally the police could fine you for driving after 30-days."

There are other various references to this if you look around and one of the best explanations is found in the pages of "Angloinfo" where this time it refers to residents not having more than 30 days grace..

"It is illegal for residents of Spain to drive a car with foreign plates, so once a car is imported to Spain, it must be registered with the Spanish authorities. If entering Spain with foreign plates, residents are allowed to drive from the port of entry to their residence only. Spanish plates must be obtained within 30 days of entry. Temporary permission to drive an imported vehicle - and temporary licence plates - should be obtained from the provincial Traffic Department (Jefatura Provincial de Tráfico) or full re-registration of the car must be undertaken.

Non-resident EU citizens
EU citizens who are not Spanish residents may drive their foreign-registered vehicles (cars, motorbikes and caravans) in Spain for up to 180 days (six months) in a calendar year as long as it is road-worthy in the country in which it is registered. In this case, only the owner of the vehicle may drive it. Students from another EU country studying in Spain are permitted to temporarily import a car for the duration of their studies, but proof of enrolment in an academic institution will be requested.

A foreign-registered vehicle may remain in Spain for more than 180 days in a calendar year as long as it is not driven. Even though EU citizens are no longer required to obtain a residence card, after 182 days the vehicle must be registered in Spain or garaged.

Note: the six month period need not be consecutive and is counted over the course of a calendar year only. This period is based on the owner's and not the car's presence in Spain."

It is a really complicated subject, but I suppose the rules are there to allow UK residents to holiday with their car, not to allow Spanush residents to go to the uk and buy a cheap car then come back here and drive around in it, so I would suppose they would have different rules..

Newchocs you have just repeated what I posted