By tax-free, I assume you mean commission free. If so, there are loads of places do this e.g. No.1 Money Exchange, Post Office etc.
However, they all give the
"tourist rate", which today is 1.102 euros to the pound. If you use your cashpoint card abroad then you get the
Visa Bank Rate which is at 1.13522 euros to the pound today. To get this rate, you will be asked by the ATM if you want to convert to Sterling. The answer is always
NO, otherwise you will get the tourist rate plus any charges the banks stick on.
Your own bank will also probably stick on a charge e.g. mines charges £1 per transaction plus 1% which takes the rate to £1.1239. If your bank is the same then take the maximum out each time you use the ATM.
If you're with Santander I believe there are no transaction charges if you use their machines in Spain.
Whoever you're with, check their charges for overseas transaction and, just to reiterate,
NEVER allow the machine (or retailer) to convert to Sterling.