Well there are different categories of P2P's. Those that block & those that shape. TalkTalk shape bandwidth. Probably by using a program such as http://www.sandvine.com/products/p2p_element.asp or http://www.ipp2p.org/docu_en.html here's an example of how gnutella can be shaped: Bandwidth Shaper Sample - typical of how some ISP's use it (click on link) ie: giving priority to net surfing & low priorities to p2p sharing.

So why do they shape it? 1. So they can give most of it to business which is where they get most of their money. 2. Because in some areas, the population of users means the bandwidth traffic is high, so to maintain a reasonable speed for those who only browse, then ... as they sometimes aim only to attract people who want fast browse speeds, ... not for heavy use such as games or p2p, etc.