Lucy,

As you will be out of the UK longer than 3 months, you will forfeit your right to NHS care in the UK. You will need to contact Newcastle for documents to state you're no longer entitled to enable you to register with a GP in Spain. Some expats never mention their extended trips to their GP's thus no one is none of the wiser and you remain on your GP's books in the UK.

The other thing to consider is because you haven't paid into the Spanish tax system (I presume you don't have a Social Security number?), your only best option is to either see a private doctor for all medical needs or take out private health insurance. Bupa are one company who offer services to expats in Spain.

HTH