Top_Gooner is correct,
I use Bt Broadband option 3 and it is on 24/7. I only have to pay extra if I exceed my monthly download limit of 40GB. There are 3 PC's on this connection downloading a lot each day, but I've never come close to my limit. I think its as near unlimited as you can get, allthough there is an option 4 for the serious downloaders.