I managed to get to Heathrow early this morning, and I thought I'd have time to get a quick update - the airport has T-Mobile WiFi, and I have a pay as you go account. It works fine in the US, so I refreshed the browser and followed the directions. Sure enough, there's a pull down menu for non-UK users, and I was able to select T-Mobile US. Logging in brought me to an unhelful "general error; try again later" message. The same thing happened last Monday on my way in, so it's not just a one time thing. Do they actually support roaming?