CSG is right of course, but, it's not always practicable, and certainly not a cheap option. Assuming you have a conventional system with a copper cylinder and a roof tank, you could have a pump fitted. This would make a great difference to the output. Another choice is to replace your mixing valve with one that has a pump buit in. Check the "Mira" website for details.
If you have a combination (combi) boiler, then the pressure will be mains anyway - with no option to boost it.
You could also simply unscrew the shower head and remove the plastic restrictor from inside. If your shower is less than, say 10 years old or so, it will be equipped with this water saving device which really reduces thepressure,. It's required by code here in the U.S. but only for selling the shower head, not for installing by the homewoner...