If Spain does go back to the Peseta it does not neccessarily mean that there will be a devaluation against the pound. The UK will take a hit too which will keep the pound low.
I do agree that if Spain continues to go downhill that Brits may not want to live there. Previously loved by thousands of pensioners the crumbling health service, Town Hall services (what little there is) will be a bit of a deterrent. Already prescription charges have gone up and where before pensioners did not pay they now have to pay 10% of costs. Adding to that almost 500 common items have been removed and patients will have to pay full price for them. Some of these are for arthritis, stomach complaints etc which affects pensioners. Hospital waiting lists have increased four fold in only a few years and surely this will get worse.