UK appliances are fitted with a three-pin plug.
Wall sockets have a switch controlling power supply.
Adapters are easy to find; if your country uses lower voltage than the UK average of 230v, you may also need a converter.
Connecting your device to the mains:
UK appliances are fitted with three-pin plugs that can be connected to the UK mains supply through wall sockets. Unlike the sockets in many other countries, these have a switch to turn the power supply on and off - make sure you've turned it on if you're trying to charge your appliance!
UK power sockets deliver an average voltage of 230v, although in practice this can be slightly higher.
To charge devices that are compatible with this voltage, simply buy the appropriate adapter from the airport or from high street shops such as Argos. If your device runs on a lower voltage, however, then you will also need a converter to stop it from over-heating. Even if your country uses lower voltages, remember to check whether your device is dual-voltage (look for the 110-240v notation) before buying a converter.
Out and about
If you've forgotten to charge your device in your hotel room, you can find power sockets in many coffee shops around the city. Some overland trains also have charging points by the seats.
Additionally, all Vodafone shops in London's Zone 1 have a Chargebox - a secure, coin operated locker in which you can charge your phone.