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SIM cards are readily available in the UK and, since the instructions are in English, they are easy to use. You can however purchase them prior to your trip. Although it will cost you more than buying one locally, the main advantage is that you have the number before you arrive and the product in hand so, as soon as you land, you can use the phone. Cellular Abroad is a an online vendor in the USA that sells UK SIM card as well as SIM cards with data. If you are in Australia or New Zealand you can get SIMs from Mobi Passport
When you arrive at London Heathrow airport Terminal 3 and you would like to buy a SIM card to get connected with local friends or talk to the hotel you have booked, do NOT buy a SIM card from the vending machine in the arrival hall unless you want ot spend 10 pounds more. Use the public telephones there instead and buy a SIM card in the city. Those SIM cards from the machine cost you 10 pounds with NO calling time while those in any telephone shop in the city have 10 pound calling time included. In fact if you stay for a few days, some SIM cards (pay as you go) only ask for 5 pounds with calling time.
Note: If you would like use the UK network "Giffgaff" you can order sims to be sent abroad for free > here (Giffgaff themselves only send out sims to UK addresses)
Giffgaff's a great network - really good value and uses the o2 networks signal so the coverage is fantastic. You can see the current prices here: http://giffgaff.com/index/pricing