RE the DL part of your post
When you drive in Spain you must have your valid DL in your possession when driving.
If you have a UK D/L and loose it then you cannot drive until you get a replacement.
If you are not resident, living in UK, you cannot renew a UK DL in UK. An address in UK is not sufficient as the form asks for the address where you habitually live, previously it was just a UK address. Any false statement regarding where you habitually reside would amount to a criminal offence.
Buzamm
Although, I believe if you move to Spain you are not required to notify the change of address with DVLC and of course you cannot use a non UK address for that, I understand that as far as DVLC are concerned, you can still use that DL, albeit that DVLC cannot change the address on the licence.
To exchange your UK DL for a Spanish one the regulations say you must have a valid UK DL to exchange. If you have lost it and then because of non-residence in UK cannot renew it, I am not sure how you can overcome that problem. Worst case would be having to take a the Driving test in Spain !
As Tadd said, all in all, if you live in Spain it makes sense to change your UK D/L to a Spanish one.