Best value post back to uk

I need to send some keys back to the uk so has anyone advice on the safest method and at the most reasonable cost?.

I would suggest that the safest way is by the official Spanish postal service - Correos. There is an equivalent service to the UK registered post, Carta Certificada.[u][/u] for valuable items.
The cost will depend upon the weight of the package

I would agree with the above reply. We had to do the same. Always sent Parcels by Spanish Post as well. Never had any problem.