Traveling to the UK for 6 months and need a cell phone? Help!?

I am traveling to the UK for a 6 month stay. I currently have an AT&T iPhone 4s, but I saw the rates for me to use it over there are really high. I'd like to know what the best prepaid phone options are? My phone isn't unlocked so I know that I cannot use it over there. I am looking for the best value for local calling and texting as well as making calls/texts back to the USA. Also does anyone know if these prepaid phones are sold online? I'd like to purchase it before I travel.