Obviously if 4 people are torrenting at once it will be really slow but just need the capability to torrent so no throttling, or less throttling
You might actually be better off with an ISP that throttles/shapes P2P traffic - that way the P2P traffic won't affect interactive traffic as much!
For the "unlimited with little or no throttling/shaping" Be would be my general recommendation for ADSL for £17/month if you can live with "up to 12mbit" (or your line is too long to support much more) or £22 for potentially faster.
For FTTC the only unlimited option I know of is the £28/month BT package though that does come with shaping/throttling, and IIRC you have to take an 18 month contract which may make it unsuitable for a student house.
In both those cases you are going to need to pay line rental on top of that price.
One important thing to note with regard to P2P activity is that saturating your link upstream can severely throttle both downstream bandwidth and interactive performance, so try to limit uploading to something like 90% of your lines maximum rate. You may need to enforce this (getting other people to co-operate with bandwidth management never works well IMO, as people tend to assume "oh it'll be fine not to bother this once" every time) by sitting a small Linux box between the router and the rest of the network and implementing a little upstream traffic shaping there.