It will cost you to withdraw £'s in the UK from your Spanish account, If you use an online transfer company (I use hifx) it won't cost a penny to transfer to or from your accounts.
At the moment I send £750 to Sabadell every month on the 1st, then on the 10th I send some back to my English account if needed, if it's not needed I leave it there for spending money when we go out to visit.