I have a Vodafone SIM I use with an unlocked Android phone when I'm over there. The SIM was free and a £10 top-up gets you 500 MB of data, more than enough for a short trip in my experience.

Three used to have a pay as you go SIM deal that came with unlimited data for £15 which would be a better deal since you'll be in the UK for two weeks. I would have done that myself but the clerks at the Three store I went to argued that my Android phone wasn't 3G. I insisted that it was. They didn't believe me and refused my business. I wouldn't expect the same problem with an iPhone.