They need to know what the cap the landlord has set actually is. Otherwise how can they stay within it. That would be the first thing that find out. If the flat mate who is trying to find that out can do so that would be great.
My power bills are based on a yearly amount of power divided into 12. So I pay the same over the summer as in the winter.
That won't work for students as they aren't there in summer. So they need to Pat As You Go type thing now to avoid issues.
I'd suggest if the landlord could give them a figure that would help them budget and anything over that sum gets paid to the landlord monthly. That way there is not a big bill. Should be easy enough to do. Landlord could be up for it, to keep out of getting into debt to students who won't pay.
Plus do they have this landlord cap on electricity too? Do any use personal bedroom plug in heaters or a tumble dryer?
Those are dear so there should be discussion about that.
Better to keep the gas central heating on than one of them buying a personal fan heater. The bills from those are HORRIFIC.
If anyone threatens that they need to be told no and if they really need it they need an oil filled type of radiator that plugs in. Not a fan.
But for that worry alone I'd suggest keeping the heating on in the flat how the others want it. Or one will end up taking measures to heat her own room which will impact the joint bills.