[GadgetKen]

When I was in the UK a couple of years ago on an Aviation museum trip, I remember the British Airways terminal my flight landed in had a sim card vending machine by the exit with multiple carriers (I used my UM+ card instead but remembered the odd machine). Probably not the best deal, but convenient if you need a card ASAP after you land. Other thoughts are a mobile phone shop (several on High St. in London and just about everywhere else due to tough competition), or buy one for a few bucks off ebay if you need the mobile number in advance of travel.