I did it at the post office for GBP to EUR when I last went over to the continent, I'm not sure if they're any good for doing it the other way though. There's also the Eurochange shop in the Mander Centre, but I've never used them so I can't really say if they're any good.
Failing those, you'll probably have better look finding somewhere in Birmingham, which is only a short train ride (or a cheaper but way longer bus ride) away.