IIRC, the Phorm technology that BT has been secretly trialling (and it's also rumoured that other ISPs are considering its use - thought hey may now wait to see what happens to BT!) merely identifies what sites you visit and then processes this data to serve up related adverts. For example, if you visit a lot of golf-related sites you will get a bunch of golf-related advertising.
AFAIK it does not monitor keystrokes, so while it might note that someone spent a lot of time in chatroom X, it will not know the conversations being had.
Besides which, there are already "anti-Phorming" programs available and I wouldn't be surprised if using a proxy would defeat the Phorm technology.