I'm heading to London for 10 days and wish to pick up a prepaid SIM for data (voice is optional but I'm willing to sacrifice for lower cost). From what I've read, I'm best to buy one in London rather than Australia for the cheapest option. Given airports are notoriously expensive, should I expect a markup on prepaid plans at Heathrow airport? Or am I better finding a 7/11 or something along the way?
Usage: 10 days of heavy email, moderate Google maps, and light web browsing.
Full disclosure: I'm a prepaid noob. Happy to take any advice :)
Three have a great plan - I don't have details handy, but it had unlimited data for a month for something like 15 GBP.