i know studentjetpacks.com offer a pack with SIM cards and other very handy stuff in..if your coming to london, they also give you an Oyster travel card! Though says for students, anyone can use them, and they specialise in providing the most economic jetPacks for the traveller.
Just checked the site for you and they say the sim is activated, so you can get your number before you come to the uk - its with Orange call abroad, costing ONLY 5pence to call the USA from you mobile to moble/landline! wow!
Cehck out
www.studentjetpacks.com/jetpacks
Dont forget your umbrella when you come to the UK!!