Virgin Mobile uses the t-mobile network according to the wikipedia entry here and from the experience of my other half, the t-mobile signal isn't great in this part of London.

Both o2 and Vodafone do very well in FH/HOP, in my experience. All of this is obviously a bit subjective, and the mobile handset you're using has a major impact. So probably worth holding on to what you've got at first and seeing how it goes.