Your main problem is that British gas are the most expensive prepayment meter providers across the UK.

Not true, it depends on the area you live in and how much others are charging, and how they are catering for the standing charge (everyone pays a standing charge, it's just more obvious on PP meters). In the area I cover BG Key meters used to charge more per unit than the biggest supplier for the area, at the moment they're 3p per unit cheaper, obviously this could change in the future.