I have Virgin and like it, fast downloads (individual mp3s bordering on instant) and solidly reliable barring one major outage that lasted 18 hours. Otherwise nifty. You are pretty much obliged to have the TV, though - trying to get a package of just phone and broadband incurs surcharges for some spurious reason and is almost the same price. Certainly they've spent the most on infrastructure over the years and most of London seems well covered - those green metal cabinets are everywhere, and lots of them are exclusively Virgin's. (Fnarr.)
I think (and anyone more informed, feel free to correct me) that Virgin and Sky (possibly TalkTalk?) apart, it's still BT's infrastructure you're using, regardless of which ISP you use. (Just as all the gas and energy suppliers use the same network, regardless of whose name is on the bill.) Therefore your main concern is going to be good value/limits. Be There looks good, don't know anyone with it but their pricing is certainly competitive. If you keep your BT landline and you're on a decent part of the BT network (i.e. they've upgraded to fibre-optic cable in your street) then you ought to achieve close to the quoted speeds.
I hope that was helpful, if dull.