Best exchange rates, and a little cashback too
TravelEx may not have a high cashback rate, but it's basically free money back when you buy at their great exchange rates. The site is easy to use and will show previous exchange rates so that you can see how the current rate compares.
I usually collect money from the airport (having bought online at a better rate), but they'll deliver for free if you spend £500+. If you spend £390+, then the cost of delivery (£1.95) would be covered by the cashback.