If they are out in the sticks try selling them a ground source heat pump and thermal store for most of their heat.
Ground Source Heat Pumps | GSHPA is the focal point of the ground source heat pump industry | Renewable Heat | Renewable Cooling | GSHP | Ground Source Heat Pump Systems - https://www.gshp.org.uk/ as for the distribution side, I would suggest make provison for 3 phase distributon to any garrage so if need be they can install pumps and so on.
As for the heating, ask can their improve the insulation, improve the controls, and tactfully suggest they need to think like what it was like before central heating, in other words in a big house you made one room tosty warm and the others were cooler.
Other ways also look at all loads that could be run off E7 / E10 The supplier should provide a time clock, or radio switch, both can appear to "drift". To make sure the switching is done at the correct time for loads which are remote, use a 6A MCB on the offpeak board to control remote contactors fed from the 24Hr supply, that way you only need to supply an extra 1.5mm T+E but mark up the contactor enclosure dual supply! or use a 240V to 24V transformer and 24V contactor coils subject to voltage drop.