And the £400 would be divided over half the amount of time, meaning the bills are very high.
Your approach makes sense if you have sufficient buffer in your current account. The poorest do not*.
Unless I am mistaken only paying for what you use and even better after you use it instead of before, is better for the poor instead of paying too much.
I didnt say it would be over half the time either.
I expect by this time next year many of the poor wont even be on credit meters, probably forced onto prepay meters due to unaffordable defaulted bills.