Firstly, BT broadband is reputed to be one of the worst deals in the UK. Why did you get broadband from BT?
What program are you using to download the torrents? I'd recommend Transmission, but I'm sure others are okay too. In your program, look in the settings to find which port it is using to transmit and receive data. Transmission tells you if this port is 'open' on your router or not. If it's not, or your program doesn't tell you, go to your router's setup page and set it to forward that port to your computer's IP address (open network utility on your Mac and you'll find it somewhere under en-something interface, probably starts with 192.168.xxx.xxx
What speed broadband do you have? There are various sites online that will check your download/upload speed - then download a really popular torrent for a while (e.g. a major Linux distro) and see what speeds you're getting.
Hope that helps.
Jonathan.