New Boiler-combi vs gravity

[SouthCoast]

Hi
I need advice I am replacing a boiler which is old and insuffiucent for my needs as I am adding 2 new rooms and a second bathroom in my loft.
In the second bathroom will be a large freestanding bath.
Lovely, but it will need a lot of hot water. The existing bath is less than 1500mm
There will also be a mixer which I would like to have decent power.
The existing system is gravity fed but has a small cylinder (was sufficient with the small bath) with restricted space for one big enough for the monster bath.
Otherwise I would keep this system and pump the shower.

My alternative thought wouild be a combi but mains pressure here was measured sub 3bar dropping to around 1 as soon as a tap is opened. Too low for my needs i would think.

Is is possible to keep the existing cylinder fed from a combi just for the shower with the rest of the system straight off the combi boiler? I was expecting to keep coffin tanks in the loft anyway and would pump the output for the shower.
The issue then is that the bath would need to be filled from tap at a time
Asking this probably reveals I am far from expert.....
Many thanks in advance.

 

[bengasman]

if your pressure drops when you open the tap, it is a flow problem, not pressrue. to make the combi work, you need to upgrade your incoming mains.
your installer will know how to solve this