There is a "Link" shop (mobile phone retailer) on the Terminal 3 departures area BEFORE security that can sell any prepay startup kits.

Your best bet is a Virgin Mobile prepay, but to each his own... Through any questions into the "travel technology" forum or "FT Travel". You could also search their for "pre pay" and I'm sure there is a reference or two comparing the UK pre pay products.

Edited to add: You could also look for a "Carphone Warehouse" near your hotel, Virgin megastore or other retailers to buy a prepay kit. Ensure your US phone is SIM unlocked (put a friends SIM in from another network and see if your phone works). There is also a "Link" retailer on the arrival end of Heathrow Express in Padington station in "central" London.

[This message has been edited by NickP 1K (edited Mar 20, 2004).]