I think you may be confusing ads for flatmates with ads for tenants. The Flatmates rate would have included an averaged-out gas and electricity bill. Long-term tenants would be expected to make their own arrangements with the energy providers or, in say the sub-let of a granny flat – pay according to a separate meter for their accommodation.
If you are renting rooms to students while you are still there, then that’s a different matter and you could ask them to pay the difference between the bills before and after their arrival, at least until you had worked out a reasonable cost.