OK, thanks, been away.
If it's an electric shower, it is going to be drawing cold water to mains pressure, prob through a 15mm supply, and heating it to a reasonable temp.
As the heating element is only able to heat a limited amount of water per second, they usually have a fairly low flow; otherwise the water would not be got enough for comfort, especially in winter like now when the incoming water is colder than in summer. I've never been satisfied with electric showers because of their low output (for example, my gas boiler is 22kw and takes 20 minutes to heat a cylinderfull of water, but an electric shower may be 8 or 10 kw and is expected to deliver hot water immediately.
If the water coming out is very hot and you would be willing to have more flow, but at a lower temperature, then have a look at your pipe run, see if it has lots of elbows and tees on it which restrict the flow.
If the water is not particularly hot, and there isn't much of it, then you would get more or hotter water with more powerful shower with bigger heating elements, though this will probably need a corrsponding upgrade to the electrical circuit. Use the temp adjuster on your shower to see that, as you increase the setting, the water really does get hotter (if not may be an element fault). Also look for markings on the shower unit to see what its kW output is; and the MCB that supplies the circuit, to see what amps it is. this will give us some clues of what might be good solution to your prob
p.s. my q about size of shower head was because I have at times used old Mira showers witha riduculously tiny head with tiny holes in it that don't pass much water, and they get blocked with limescale which cuts it down even more. Try undoing the shower head and filling a bucket straight from the hose at high temp, see how long it takes - this will give you a maximum possible flow.
