I am traveling to the France and the UK with an Unlocked Verizon iPhone 4s. I have a 4 hour layover at Heathrow before continuing to France for the week. I will be returning to London for two weeks after that via Eurostar.

I use a fair amount of data on my iPhone. I am looking for the best options and places to buy my microsim card.

Giffgaff seems to be a good choice, but they will not ship a simcard to the US and I cannot seem to find out if they are available to buy at a store.

What networks would you recommend and are their any stores in Heathrow that sell micro sim cards for phones. I will be in Terminal 5 as we are traveling on BA.

Thanks for the help...