Originally Posted by ScottC
Yes, Virgin is GSM, and yes, they DO roam on many other GSM networks, including some in the US. Just because Virgin in the US is on Sprint doesn't mean they can't make use of roaming on another US GSM network.
Virgin Mobile UK rides on top of T-Mobile's GSM 1800 network in the UK.
http://www.gsmworld.com/roaming/gsminfo/index.shtml
Indeed, you can pick up a Virgin starter kit at Virgin Megastores or at Cellphone Warehouses, which are about as ubiquitous in the UK as McDonalds franchises.
If you're really hung up on getting a SIM with with Top-Up card before leaving the US, lemme know with a private message--I may have a Virgin starter kit still in box with 5GBP on the card.