I really fail to understand your banks attitude - I know lots and lots of people who are resident here and retain their UK bank accounts and also many people who remain resident in the UK but have Spanish bank accounts!
I suggest you return to the bank and ask to speak to someone in higher authority who knows what they are talking about. I would suspect to say you cant is actually against EU law.
On a point regarding my original posting it does appear that some banks/building societies will not agree to pay interest with gross of tax to non UK residents - the Inland Revenue form does say that it is at the banks discretion and it not a legal obligation. But its worth asking.