Ok so for you the best case scenario is that they let you pay for the energy in a manageable way as one way or another the energy has been used, despite you potentially thinking you were using less than you actually were. (as you acknowledged)
So agree a payment plan with them, if they won't agree to something that you feel manageable or fair then make a complaint (assuming you've already done this and had an outcome you dislike) you need to speak to their exec complaints team and if they fail to help you need to take it to the omsbudsman.
They will usually try to resolve something before the omsbudsman comes in because it costs them money.
https://www.britishgas.co.uk/busines...unt-complaint/