Rates change so often that it's unlikely that you'll get a definitive answer. However, here's a quick summary of the advice that's usually given in response to similar questions here on AB:
1. The best rate is probably achieved by using cash machines, at your European destination, with a card which doesn't incur a charge when used overseas. The best known card is the Nationwide FlexAccount card.
2. The second best rate is probably achieved by using any other debit or credit card at overseas cash machines. (The small charge which your card issuer will make is more than offset by the good exchange rate).
3. If you're determined to get some Euros before you travel, the best rate is often to be found at those branches of Marks & Spencer which offer currency exchange.
4. If you're not close enough to a suitable branch of M&S, your local post office is probably better than any bank, travel agent or bureau de change.
5. If you're completely mad (or you simply enjoy throwing your money away), you should change your money at the airport (or rail/ferry terminal) on departure.
Personally, I use a combination of my local post office and overseas cash machines.
Chris