Probably that's the root of the problem. For any pressure in the system you've got, the feeder tank has to be higher than the shower head (but I'm guessing you know that) - and the higher the tank the better the pressure. In my house, before we changed the system, the feeder tank was in the loft and raised by polystyrene blocks resting on the joists, so that the water level in the feeder tank was raised. It's all to do with the measurement between the level of the top of the water in the tank and the shower head. It should be a minimum of a metre.
Check this out:
https://www.tradeplumbing.co.uk/how-...your%20area%20.
You'll see some fixes here for the problem.
Raising the level of the tank is easy, as I've described, providing you've got the space in the loft.
Fitting a pump is just a matter of connecting the pump to the hot water feed and making an electrical connection. Simples.
Or you could just fit an electric shower, connecting to the mains.