I’ve had the same problem trying to change bank account in the UK while living in France, they all want you to sign something saying you’re resident in the UK and I’m not (and I’m not sure if spending two weeks back in the UK over Christmas really counts as “resident”).
I do know that HSBC will allow you to open a bank account abroad using your UK address temporarily until you have a foreign address sorted, as long as you have a UK HSBC account registered to that address too. I’m not sure if they’ll let you do this from the UK, but it’s worth popping in to one of their branches and asking.
Edit: Just re-read the OP and looks like you already have a Spanish address so that won’t be a problem. I would still try HSBC as they seem to be the most international of the major banks, but try Santander too as they’re a Spanish company.