Sending important ducument to UK

I need to send a very important ,original, signed and witnessed document from Quesada to a solicitor in the UK.

For normal letters I would use one of the excellent Post Box shops, but given it's importance to me I'm wondering if anyone has done this recently and whether they could recommend any other carrier or such like. Or are the Post Box shops the best bet?

Thanks in advance

Hello we used Correos registered and everything was tracked. Other than that, the usual DHL or similar.

I would use registered and signed for with Correos

Thank you both for your very helpful replies.