Apart from the fact that it is a Spanish Legal requirement, even if you used to work as a civil servant, teacher, soldier, sailor, RAF pilot, nurse or in any other government employment, which require tax to be paid in the UK, you earn longer term benefits.

The Spanish tax system , understandably, just as the Spanish inheritance laws and capital gains tax laws, were devised with Spanish residents in mind. These laws do not fit comfortably with anyone arriving in Spain from the UK.

YOU COULD EVEN GET A REFUND OF UK TAX

There are two distinct types of residency, one which entitles you to a green A4 certificate that says you are a resident  ( on the national police register) and "FISCAL RESIDENCY" which says you are entitled to certain tax breaks.
These consist of higher IHT allowances and exemption from Capital Gains Tax, if you sell your home and are over 65 when you do it.

You need to get this checked out, please go to www.taxspain.co.uk to find out more