For multiple locations electric showers are bad news. A decent one will cost £300 + fit and then need a 45 amp / 10 or 16mm cable run back to the fuse board.
2 units running 40 amp demands simultaneously could pop the head fuse, which is normally only a modest 60 or so amp.
The combi + tank is perfect, but does require decent mains pressure- possibly an upgrade to 22mm pipe from the street in would resolve the pressure issue.
You might have a reduced bore due to chalk build up since all London suffers from limescale build up- if you taps and kettle suffer then the mains pipe will unless it's plastic.
My understanding is that simple use of a 3 way valve on the combi h/w out will allow the topping up of the megaflow tank. That can be set with timers and also as a constant temp controlled demand via a thermostat.
The beauty of the set up is that the simple demands of water for the kitchen sink, washer and d/s cloak are direct via the combi and only the baths, bath sinks and showers would run via the megaflow tank.
With a combi it seems to be the only solution getting decent flow to the showers.
Not cheap and at the total cost (??? £1500) is well within the cost of getting a brand new combi and far better output (some go to 19 ltrs a minute now).
Either Way the mains pressure needs sorting 1st.
PS I'm just a spark, who has simple knowledge such setups- I've not done it, just heard that's the way to go
The megaflow unit will also have an element for direct water heating should the boiler fail