The Coin Shop, Ship Street.
I would go to ship street and change a small amount,then change the rest when you are away as at the moment other countries pay good euros rates for the pound.
Example:Got 118 at ship street,when i arrived in Mollorca i got 129.
I've always found taking as little as possible, then using your maestro/visa debit card in a local cash point to be the cheapest rate of exchange for Euro's. Check with your bank, but done this way there is no commission, and usually a good rate (at least that is what I have found).
Just remember you will pay a charge each time you withdraw cash to the foreign bank and maybe a charge by your bank as well, you could be charged up to £5 for each withdrawal with the two.