Expansion vessel water expands when heated, so either a header tank or expansion vessel is required. Likely for domestic hot water not central heating.
Yes, and here's a pic of the consumer unit (one of them):
That seems like the supply to storage heaters, likely you can switch off whole consumer unit and all lights and sockets will still work.
I had a storage radiator at work, it was to evaluate the bricks we were making for them, started using concrete instead of clay, it was not a success, as when heated for the first time the iron ore changed state, and it caused water to be produced which ran out of bottom of storage radiator, the idea had been pressing bricks rather than baking them would cost less, but had to bake them any way to remove water.
However it was turned off, set to no output, but to keep the energy inside the unit at 900°C is simply not going to happen, it still heated my office.
And that is the problem with the storage radiator, you can alter how fast the heat leaves it, but can't stop it. There have been some systems with a central brick store which could retain the heat for a week, and also large water stores, but they take up a lot of room, so not really an option as an after thought.
There was a control for how much heat goes in, hotter the bricks the more heat it can give out during the day, cooler the bricks less heat it stores. but it was the trying to estimate how much heat is needed next day which means they don't really work.
Not seen answer is she on a duel rate, as said so many still have the heaters but have got rid of the duel tariff.
If not on a smart meter or a duel tariff not sure how they know power used over night, however it could be she still has two meters even when she no longer has split tariff, so that is still the big question.