ISPs inspecting traffic for illegal filesharing ..
..would be an interception under RIPA, and highly illegal.
ISPs can look at traffic for purposes connected with the supply of their service. This is normally taken to include virus scans and spam filtering, though there is some question about the latter - but it certainly doesn't include inspecting traffic for illegal filesharing.
What the music/movie biz lawyers usually do is look at publicly available bittorrent information. Whether the download itself is encrypted or not doesn't matter much, it's this publicly available information which is used.
I am of the opinion that the music biz has it's knickers in a twist mainly because revenues are falling - and the reason isn't so much filesharing, though that has some effect, but simply that people do not listen to music as much as they used to.
The movie industry seems less vocal about filesharing, even though percentage wise I'd guess the amount is similar - but people aren't watching less movies.
Another factor may be the cost of downloading a DVD, which is significant, compared to downloading a music track, the cost of which can be ignored.
BTW, a DVD in the post is a much cheaper way to send a a movie than bittorrent - a blank DVD costs 20p, postage 29p, sleeve 2p, so it costs 51p, or two for 71p. Downloads cost about 40p - 90p per GB (you _will_ pay this, in one form or another), so to download a 4 GB movie costs £1.60 - £3.60.