Hm, i am with Virgin Media, i recently went from 4mb to 20mb, i dident notice a increase in speed when i was on 20Mb. Although i was uploading alot faster. I was not satisfied for what i was paying. so now i'm back on 4mb.
When NTL turned to Virgin Media, i got upgraded from 2mb to 4mb for free, and do not pay anything more than i used too.
I think Virgin Media could do with more speed choices anyway, there is only 2Mb, 4Mb, and then 20Mb.