Metronet is a PAYG Broadband provider but they do have minimum charges depending on the BB speed you choose. There is also a maximum usage charge as well. But the setup charges are quite high to start off with. They also have a 1 month minimum contract as opposed to others who have a 12 month minimum.
You could do a lot better using a standard BB supplier with a fixed monthly amount (most, if not all, give you a free modem, free connection).
Some are capped and some are not.
Have a look at this site: click here
I am on NTL cable telephone, TV, and Broadband, might be worth a look if you have cable near you.
You don't have to have NTL TV or telephone, you can just take their BB service.