Although the energy suppliers like to give the impression that they are entitled to set the monthly payments to any figure they want, even when this results in you giving them an interest-free loan, I understand that in fact they do not have this right, and you can refuse to accept their proposal. I now counter with a curt email saying what the monthly payment should be, and I find they agree (until the next bill, when they try it on again). I found BG were the worst.
I also maintain accurate monthly readings and the 12-month rolling usage.