Deep packet inspection can tell an ISP a lot. I don't know all the details as I don't deal with that side of our system (I'm more the domains and hosting systems end of things), but I do know we can shape broadband traffic to quite an impressive degree here - and we're a hell of a lot smaller than Virgin.

A quick test would prove it - get everyone in the house to stop using all forms of Torrent or P2P (other than the WoW one, which is almost certainly detected and classed as legit) for a full week, and see if the problems go away.