If you are visiting London with friends or family, and need an inexpensive way to stay in touch, it is best to purchase local sim card to avoid international roaming charges.
You need the service called "pre-paid" or "pay as you go". There are several major mobile phone carriers in the United Kingdom. Consider purchasing pre-paid cards directly from the operators, not from resellers.
Large carriers are:
The rates for pre-paid packages are similar with all providers. To get an idea of what the pre-paid sim card rates are, please check these online:
You can find retail stores of these carriers throughout London and purchase the service directly from them. Later on you can always refill your account balance online if you choose to do so. If you need an exact location of the retail store, please visit the carrier website and use "Store Locator" section to find a store that is conveniently located for you.
Similar to pre-paid sim cards for your phone, you can purchase a pre-paid data package from the key mobile phone operators. Same companies as listed above would be happy to provide you with pre-paid data options.
Check out the example of plans for tablets from Virgin Media.
Please read instructions carefully when you sign up, as some plans get auto-extended and you need to unsubscribe from the service when you don’t need it any longer. Otherwise mobile operator might keep charging your card.