The best rates for getting Euros are usually obtained by using your debit card in an overseas ATM. Even though you'll face some charges for doing so, you usually get a better deal than by using Bureaux de Change.
On the High Street, Marks and Spencers is often amongst the best places to try but, given the time (and fuel) you might use up trying to get a marginally better rate, the Post Office is possibly the best place to go.
Try Travelex. Their rate isn`t bad. For £3.99 you can get the buy back guarantee. You can change back into sterling at the same rate you bought at. Eurochange`s rate is OK but they have percentage limits as to how much you can change at the buy back rate. M&S do a buy back but they are a bit cagey about the charges and give you a different rate so I can`t really see the point.