When I got to Vegas I discovered the exchange rate was better at the cage in the Aladdin than what I got at the post office in London. Also, when changing dollars back into pounds, try the casino in the UK, they tend to offer a better deal.
If you are still in London I can recomend a good place. It is on Victoria road and is primarily a ticket tout shop. Goes by the name of Access tickets +44 207 828 2747, alternatively try Amex in Wilton road (Victoria also) +44 207 630 6365. Not all Amex outlets give the same rate but you could try your local branch.
If you are in Vegas already, try phoning the cage at the Trop or the Luxor, one of those two has never been beaten when I have been calling for rates, althpought they have never matched access tickets !!
As for changing back into � I have no idea who gives the best rate, sorry.