Delaying the fill on a loft cold water tank - electric and mixer showers running at once

[tryingmybest]

Hello, and thanks for taking the time to read.
I'm trying to work out if the idea I have is practical?
The water pressure in my newly renovated house (4 bathrooms is tragic).
Current set up is:

• A hot/cold mixer in the main ensuite which is ok as long as you don't run anything else.
• One electric shower 2nd ensuite, cold mains fed - even on its own the pressure is sad, with both showers on a dribble.
• Main bathroom, has the wire for electric but not yet been installed. (on the other side of the wall from ensuite 2)

I have an idea to put a large cold water tank in the loft to gravity feed the two electrics using something like a couple of 'Mira Elite SE Pumped Electric Shower - 9.8k'.
While in 'theory' this would supply both the electrics to run at the same time (I think..) - but if the tank starts filling up straight away that would affect the mixer shower cold feed.
To ideally run all 3 showers at once, if the loft tank is big enough, is there a way to say delay it filling up for 5 minutes. (ovs don't want the water supply to run dangerously low while the 2 electrics are running as mega dangerous)
Looking around, I've found things like:
Keraflo Aylesbury K Type Delayed Action Float Valve - to use with the water tank
Is that what I'd need? Although pretty expensive for my budget.
Sorry for the long mail.
Any advice, gratefully received, even if just to say - what you're trying to do is insane (o:
Many thanks,
L.

 

[Ben-gee]

Have you got capacity in your electrics to run two 9.8kW showers at the same time, along with your other household needs?

 

[tryingmybest]

Have you got capacity in your electrics to run two 9.8kW showers at the same time, along with your other household needs?

Hi Ben-gee.
They're wired to separate switches on the main board, so I'm hoping so.
The whole place has had a rewire and a new main switchboard popped in due to the size upgrade.
We've had a couple of sparks in over the past 12 months and neither have mentioned a potential issue.
Thanks,

 

[tryingmybest]

Hi Ben-gee.
They're wired to separate switches on the main board, so I'm hoping so.
The whole place has had a rewire and a new main switchboard popped in due to the size upgrade.
We've had a couple of sparks in over the past 12 months and neither have mentioned a potential issue.
Thanks,

But even, the worst case and we could only use one electric at a time. Is there a way to delay the fill on the electric feed tank, so it wouldn't affect the mixer shower?