By UK I assume you are able to chanfe your cash in central London.
The best places I have seen are a combination of Thomas Global Exchange, ACE-FX and Best Foreign Exchange or ICE. To avoid traipsing around different places, go to the Travel Money section of moneysavingexpert.com - it's usually pretty accurate at who is giving the best rate on the day. Forget about Hight St banks, the post office or walking up to other big brand bureaux de change - you are likely to find yoursely stung with some very hefty fees.
Having said that, changing USD or EUR into sterling will get you a good rate at one of these places, if you have cash in a currency for which there is little demand in London like Chilean pesos, you will find it a little more difficult.
Last edited by davidjohnson6; 4th February 2012 at 03:34.