I don't know about Pipex, but I know Freedom2surf have a members area homepage where you can keep an eye on your bb useage... Pipex prob. have same sort of thing.
Word of warning... 1GB is really not that much given it includes all uploading as well as downloading. Studies have suggested the average BB user will use around 3 - 5 GB per month (and that's not the gamers!) But if your only checking e-mail and never downloading programs, 1 GB is prob. okay! :)