BT can very easily identify when you are using a torrenting client because the packets are P2P. BT doesn't like P2P packets because they are known for, of course, file sharing. Most people who use file sharing tend to download very large files and as the BT network has grown (like ALL other networks too), they have put into place procedures to reduce the effect heavy file sharing has on its network. This means that during the evening on week days, and all day weekends, BT will heavily restrict your P2P access so that it doesn't clog up the BT network and slow down other people. If you persist at trying to download P2P at this time, you may also find it isn't just P2P packets that are going slow. On the plus side, this restriction helps you when you want to use things such as Voice Over IP as your neighbour's P2P activity won't cause any lag in your speech because you will have priority.
If you really want to P2P, look into buying a SeedBox somewhere in Europe. This is like a server that only runs Bittorrent which you can send Torrent files to so it can download P2P for you. Then, once it has finished, you can download them through your internet browser. This counts as HTTP packets, which BT will not restrict, unless you really heavily abuse it (i.e. 250GB + per month). Seedboxes go for as little as £10 per month.
