If you are resident in Spain in excess of 183 days in a financial year you should pay Spanish tax- You cannot be taxed twice on the same income - Double Taxation Treaty - read the UK govt tax pages.
The Spanish will not chase you to pay tax here (unless of course you start a salaried job in the system then its automatic)
If you come into the Spanish tax system it is 18% tax withheld on interest earned on a residents account. But you might want to think twice about moving sterling into Euros at the moment!