I've used BT Broadband for many years now, and overall I am very impressed by the speed, reliability and price.
I actually downgraded to Option 1 a year ago, reducing the bill from £29.99 to a much more respectable £17.99 a month. At that time my bandwidth usage was not too high (consisting mainly of online gaming,) but even though I probably exceeded Option 1's meagre 2GB limit regularly I never received an email from BT's so-called FUP (Fair Usage Policy) people.
Recently however I have developed something of a torrent habit, and I discovered that BT do send out FUP emails, but only when you exceed the limit by a considerable amount; in my case by 46Gb :)
'Time to look for an unlimited account,' I thought. 'Shame that BT don't offer one.' Their premier product, Option 4, is capped at 50Gb for £29.99 a month.
But when I read the FUP email a second time it suggested that I upgrade to Option 3, which offered unlimited usage.
After a bit of checking around it seems that BT are abolishing Option 4 and making Option 3 unlimited for £26.99 a month. This is not yet reflected on the BT website, which still states that Option 3 has a 40Gb cap, but rest assured it is the case.
You can read more about it on Digtal Spy.