Check out the attached link to find where your local base station is.
Be aware that although T Mobile have merged with Orange Virgin still only have access to the T Mobile network. Virgin haven't yet said when or even if their customers will be able to take advantage of the better coverage offered by the new network although I've seen a few posts across different forums suggesting some sales staff are saying it will become a reality.
Anyone from VM care to comment?
http://www.sitefinder.ofcom.org.uk/