Run your cold water off the mains.
Hot water via a high pressure storage system = to mains pressure ( this would be much better).
If you have a bath, then you will need to store a lot of hot water.
You could try an in-flow water heater ( heats up the water as you use it) but the water flow rate will be slow.
Is your problems due to flow rate or water pressure?
The header tank on your gravity feed system. whats your header hight? ( the hight the bottom of your header tank is from the highest tap/shower outlet).
can you increase the hight of your header tank?
Please provide more info, for better answers:-
your shower, electric or mixer?
do you have a bath?
header tank size and header hight?
Can you install a bigger header tank? (then pump)
How many bathrooms?
What is your current flow rate? Hot& cold reading.
What are your goals? better shower flow rate or pressure or ?
(plumber).
p.s normal for a pressure reading to be zero when a tap is open.