If you look through previous threads and other sites on the web eg anglo info .com you will see a vast amount of information and differing opinions, far too much to go into here.You will soon see that some think Spain is the best thing since sliced bread, others are more realistic . I am surprised that people are still so keen to want to move here, unless they are very wealthy. I would sum up my advice as follows:
Get your info from the horses mouth first, then look at the personal experiences of others before making your mind up.
Contact the DWP, HMRC, They advise re any allowance which may or may not be payable in Spain; tax can be complicated as if you live here over 183 days you will be tax resident here. You will find that people give you contradictory advice, so go to the experts.
Don't do as some do, and attempt to "live under the radar"
Health care is the most important consideration, you will need private health insurance, unless you are going to work here and pay into the social security system, but any existing conditions will not be covered. Dental care is private, and though cheaper than UK private care, a lot more expensive than NHS dental care.
Schooling-loads of info on EOS. I think people are not prepared for the expense involved-I am not talking about the private schools here. moving children here at secondary school stage is quite disruptive and unsettling even if they do settle down and become bilingual. Some have to go back a year. It is easier when they start from primary stage.
It's a huge undertaking-Good Luck!