Unless you really need the cash it would be best to simply hold onto your Euros until your next Euro-zone holiday. (I've always got Euros in my wallet).
If you're determined to exchange them for sterling, your best option would be to find a friend, relative or colleague who is about to visit a Euro-zone country and offer him/her a rate which is slightly better than he/she will get elsewhere.
If you've got the receipt from when you purchased your Euros you might find that the place where you purchased them from will buy them back at a better rate than they give to others.
Otherwise check out the Post Office or Marks & Spencer. They generally offer better rates than places like Travelex.
Chris