This question comes up frequently. There seems to be some consensus that the best way to get your Euros is to use your debit or credit card in a Spanish ATM. (You'll get the best deal if you've got a bank account which doesn't charge for overseas withdrawals, such as a Nationwide FlexAccount, but even allowing for any charges it's still probably better to use an ATM than to use any UK bureau de change)
However, that doesn't actually answer your question. If you want to get some Euros before you travel, the most frequently recommended source is Marks & Spencers. If your local M&S doesn't offer exchange facilities, the next most popular place is probably your local Post Office.
Chris