OK, here's the scoop.
As mentioned above, check the rating plate on your electrical device. The chargers for many items like cell phones will probably show a voltage rating of something like 120-240. In that case, all you need to do is get an adapter for the plug so it will fit the UK socket.
If, on the other hand, the device only shows a voltage rating like 120 you will need to get a voltage converter which will take the UK 240 volts and throttle it down to the 120 volts your device needs. The trick is to get a converter that will handle the load your device will demand. Converters are rated by the wattage they can output. You will need to know the wattage your device needs, and purchase a converter of AT LEAST that wattage output. In the case of devices that have electric motors you will need a converter that outputs at least 20% more wattage than your device calls for.
If your rating plate does not show wattage demand for the device, it will show amps. To calculate wattage, multiply voltage times amps. For example, a device that requires 120 volts at 4 amps would need 120x4=480 watts, so you need a converter that supplies at least 480 watts.
Here is a link to a 500 watt voltage converter from a US company that I bought my converters from:
http://www.voltageconverters.com/itemdesc.asp?ic=VC500W This one costs $30. A 1000 watt model costs $60.