Low Pressure in the Shower: Causes by System Type
Gravity, combi or unvented, the fix is completely different depending on what you have.
Know your system first
A cold tank in the loft and a hot cylinder = gravity: your pressure is set by the height of that tank, and no shower head will change physics. A combi boiler = mains pressure: the boiler's flow rate is the ceiling. A sealed cylinder = unvented: mains pressure throughout, so a weak shower means a fault, not a design limit.
Cheap wins in order
Descale the shower head (overnight in vinegar). Check the isolation valve behind the shower is fully open, half-closed valves after past work are absurdly common. Clean the inlet filter on mixer showers. Only then think about pumps or boiler upgrades.
The right big fix per system
Gravity: a twin impeller pump or a move to unvented. Combi: check the boiler's flow rate against the shower's demand before blaming pipework. Unvented: a pressure-reducing valve set wrong or a failing expansion vessel, both engineer jobs, both quick.