the way i always understood it, every action taken against a p2p user has been on the basis that an organisation working on behalf of the copyright holders (typically major record labels and movie studios) used torrent/p2p to download copyright material from the user (at this stage the user is only identified by IP and possibly screen name). they then used this as evidence to support their request from the isp for the details of the user from the ISP, send a cease and desist letter, or, in the case of other p2p where they have a list of copyright materials shared by the user, threaten legal action. I'm not even sure that any of these cases ever made it to court in the uk, they were all settled (who knows for what amount).
rapidshare is different. when you click to download a link, you may not know what it contains. the isp knows, but, they have no reason to think as a matter of course that you would download illegal material, and they will not scan the packets on spec at the behest of copyright holders without evidence of wrongdoing. as third parties are not able to see you click that link, i dont see how a copyright holder could lawfully gather the evidence needed to get the isp to reveal your details. I am unsure about the policies of file hosting services beyond that they remove copyright material on request, and that, obviously, they have an ip address linked to the uploaded file.
tbh it's a year or so since i looked into all of this, if u are really concerned, i'd advise trawling through slyck.com for any relevant developments.
p.s. i agree that the ISP's most obvious concern is bandwidth usage, but just using a lot of bandwidth will at worst lead to you losing your provider, not to them looking for evidence of illegal activity