the best way is to use a currecny broker. As you are taking your money out of the country the Spanish banks will charge you a fortune. A friend of mine recently transferred 89,000 euros to her UK bank account and was charged 3,560 for the privelege. When she queried this at the bank they just shrugged their shoulders as if to say – why should we care
There are a few around such as currency uk, currencies direct and HIFX. All offer the same sort of service, all are very geared towards custmers and all charge the minimum. You can google for all of these and there may be more. I personally use currency uk but there is little difference between them all.
What I would also sugges tif it can be done and the buyers are UK based, is that they transfer the moey direct to your bank account in the UK. Obviously you will need to ensure this is done before you sign over but it means they then take the cost of transfer and if it is being bught from uk funds there is no transfer fees. You just need to be careful about it though.