Ah now you tell us ... eek

Have a look at the TalkTalk reference here ISP's known to BLOCK or Filter p2p or specifically Limewire

I believe there is a partial answer, but it's basically to use it only during certain times of the day. That's from memory.
Alternative is to find a UK ISP that doesn't filter. You can send ukbobboy01 (click on link) a personal message to ask him, as he has done some research in the UK area for ISP's that don't shape bandwidth.

And yes, it's not unnormal to be using p2p fine & then suddenly speeds suck. When the ISP detects you using high bandwidth & p2p then they will apply the brakes.