Hi Pete,
Recently the Spanish by royal decree have abolished the need for a residency card for EU citizens, that used to take up to 3-4 months and several visits to a foreigners police station to acquire. Now you are only required to attend the police station and announce you wish to be a resident to receive a piece of paper that confirms such.
However it´s sounds as you are really in no hurry to become resident yet. You could stay in Spain for 6 months before you take the desicion.
There are thousands of Brits who have lived full time in Spain for years without becoming residents. Around my Costa they just drive UK registered cars with top boxes on looking as though they are on holiday!!
The law however requires you to become reident if you plan on spending more than 6 months at a time in Spain.
There are several factors that could help you decide.
1) Try it for six months first before applying for residency as you may not like Spain or your frequent hospital visits may become too much and you decide to go back to the UK then at least you have not commited to anything.
2) During the six month´s speak to a lawyer or accountant in Spain before making any moves.
3) If you keep a UK address and bank account I´m sure you can delay the decision for a few years before commiting.
4) You could bite the bullet and after six months register for residency. If you feel the need to return to the UK to live it´s easy to cancel (many don´t bother) but better to do so and nothing is lost.
Don´t worry over the desicion too much Pete, it´s really not a problem, Spain could relax all the laws tommorrow and negate the need for it altogether, many people expect that to happen in the future.
I hope it helps but there are loads of people here who can offer further advice.
Good luck
Gillespie