I don't know which ISP you work for, but I've not heard anything good about Pipex for a long time, and it seems Tiscali buying the company has only made things worse.
Aquiss have a good AUP policy, if you go over your limit, you will be charged at £1 per additional giga plus an admin charge of £5 (this I believe is standard entanet polcy anyway).
But as other contributors to this thread have stated, you pay less, you get less.