Using electric showers would be a mistake.
Do you see a travelodge using electric showers?
You need to keep running costs down and that means gas for cheap
hot water.
As previously stated you need to measure the incoming flow rates
and analyse what demands you may need.
Unvented cylinder(s) and accumulator tank(s) most likely necessary.
Consider acv unvented cylinders very fast reheat times and will
drag as much as the boilers can supply reheating water.
Double glazing. No use getting sentimental.
Wouldn't worry about insulating above the normal.
Zone the rooms so that you can control heating to rooms not in use
and therefore save heating costs.
Thanks DCawkwell. OK, so the message is clear to steer away from 6 electric showers, and whatever way I need to check flow rates and increase pressure as necessary. If I go for a system then, I like the idea of the tank-in-tank ACV cylinders: probably have two for the 5 showers and then perhaps have just 1 electric shower in our area for redundancy.
I was going to put Drayton TRV4s everywhere bar 1 rad, but would I still need to zone?
If I went for a very large combi, I saw a mahusive one that come out very well on the Which? tests, a Worcester Greenstar Highflow 550Cdi. It says it is very good for very large detached houses, or much older period properties with poor insulation; also has low minimum output for heating when needed but has a lot of extra power to heat up rads quickly (7.4 - 29.2kW and 89.2% efficient). However, while hot water heat output is a massive 44kW / 20 litres/min, the water efficiency is just 49% - not sure how expensive this would turn out to be for heating water. Is it worth considering this as a part-combi option?