I'm going to London tomorrow for two days. And since i need to use my iPhone 4 for data while i'm there i'm gonna need a prepaid card. I've previously used O2 and their "unlimited data for a month for £10" offer. But since i have to get a micro sim anyway, i might as well switch provider. I've heard that there may be some other offers out there that is cheaper, like virgin, but i don't know if they can supply a micro sim in store. I won't be using huge amounts of data. Mostly twitter and google maps + some web. Any suggestions?