Since arriving back in the UK and getting a my phone back online I've been using GiffGaff. They rock. It's the most value centric "pay as you go" Smartphone talk 'n' data plan that I've used in the UK yet. Oh… why? Cos' like they give you a decent unlimited 3G/GPRS mobile web browsing service each month with a minimum of a ten quid top up, as well as a whole bunch of free text messages, GiffGaff to GiffGaff network calling and 100 free UK calling minutes.  However I couldn't get the bar lifted to look at adult stuff despite trying to submit my passport details as proof. From the logo that pops up it appears to run on the O2 network and the standard sizes sim card is also pre-cut to function as a micro sim too for iPhone 4 and iPads. The overall top up pricing structure for GiffGiff kind of looks like this... (official link here with a less miniature info page)