Once your funds are in the Spanish bank, they can be repatriated to the UK overnight. I thoroughly recommend that you open an account with wise.com (formerly transferwise). They will save you loads compared with the rip-off fees charged by banks. (I've been using them since 2013 and have never had a problem.)
In essence, your euros are transferred to an EU branch of Wise, normally free of charge, and are immediately transferred on to your UK Wise account in pounds. They charge a fixed percentage fee which is around 0.5%, and no transfer charges.
The next day you simply transfer your pounds to your UK account.
If you don't already have online banking in Spain, I'd suggest trying to get it. Most Spanish banks have an English language option for their online systems and these can be accessed either from Spain or the UK (or almost anywhere.)
You don't mention how much we're talking about. There are limits (usually per day) on how much can be transferred by online banking, so for a large sum, it might be preferable to give your bank a written transfer instruction.
Wise have very helpful people and they can help you. Normally on-line, they give you the IBAN details where the money is to be sent, along with your reference code. These are the details to give to the Spanish bank, whether online or on paper. As far as the Spanish bank is concerned it's a transfer of Euros within the EU and should be free, although that depends on the exact terms of your account.