The vast majority of countries use electrical systems which supply electricity in the range 220V to 240V. (The UK has a nominal 230V supply). Travellers from any of those countries can use their appliances in any other country, simply by using a cheap adapter which allows for the various types of plugs which are used.
The USA (together with a very small number of other countries) has a 120V system. Plugging a US appliance directly into the the UK supply (using a simple adapter) would result in almost immediate (terminal) damage to the appliance.
You can't simply use an adapter plug. Instead, you need to use a device which incorporates a transformer to drop the voltage to that required by US equipment. Such devices are more expensive than simple adapters. Further, you have to ensure that they have the appropriate power rating, otherwise you'll simply burn out the transformer.
Maplin Electronics sell what you need. If you've only got low-power US devices (such as mobile phone chargers) a 45W converter might do the job (�17.99).
http://www.maplin.co.uk/module.aspx?TabID=1&cr iteria=usb&ModuleNo=19413&doy=search&C=SO&U=St rat15
A 100W model costs �29.99, the 200W model costs �34.99 and the 300W version costs �39.99. (Unless you've got a lot of US devices, or if you require more power, it might be simpler and cheaper to buy UK electrical devices).
You can find your nearest Maplin store here:
http://www.maplin.co.uk/StoreLocator.aspx?
Chris