If you are using La Caixa's Lineal Abierta on line system the charge should not be very high.
I transferred a similar amount from La Caixa to a Deutchebank account and the service charge was just 3 euros...
This transfer was though, wholly within Spain and all in Euros, so there will be charges relating to the exchange rate for transfers into a UK account. There shouldnt be that much difference though because its all done using the IBAN system.
You will need the IBAN number of your UK Bank Account to carry out the transfer and the beneficiary name which presumably will be the accunt name. You will also need your Linea Abierts Code Card because at some point you will have to refer to a line number they quote to key in the corresponding 4 digit code.
As I recall, during the online form completion the charges involved are displayed before you actually proceed with the transfer so you could always stop the transfer if the charges are too high.
Hope this helps...