I'm not sure why some plumbers seem to think this is rocket science. You have hot water and cold water being fed, both at "mains" pressure to the, shower. The hot water may well be at a lower pressure because it may have been reduced prior to the boiler or by the boiler itself, but it should still be under more pressure than you'd get from a tank in the loft. You can get away with almost any shower if you can rely on other people not demanding water while you're in there.
The difficulties arise because a combi boiler is limited in how fast it can provide hot water. The flow rate is typically in the range of 10-15l/min of water heated up by a certain amount. If someone else in the house uses hot water at the same time then they get a proportion of this flow and the shower will turn dramatically cold. To a lesser extent, it can go hot if someone uses cold water. In winter, the temperature of the output hot water may drop if the flow rate exceeds what the boiler can heat to your set temperature, and you would see this as lower shower flow.
The solution to these problems is to get a combi boiler powerful enough to support your shower fetish and to have a slightly intelligent shower. The 24kw power level is just about adequate (I had 20kw for many years and was quite happy with the shower), 30kw is good enough for almost anyone, and higher should only be needed for mega-shower freaks or when you know other people will be using hot water at the same time. A power shower generally pulls more water than any of these "one bathroom" combi boilers could supply.
There are two standard types of intelligent shower: the pressure balanced and the thermostatic. A pressure balanced shower is usually good enough for a combi boiler and automatically adjusts the hot or cold supply if the pressure from the other one drops. If someone turns on a tap elsewhere, you continue to get water mixed in the same proportions and hence more or less the same temperature, although the overall flow will drop. The thermostatic shower does the same thing, but it more precisely mixes the hot and cold to maintain a constant temperature. A thermostatic shower works better when the water input water temperature may change, for example if the boiler flow rate becomes too high when someone turns on a tap and the temperature drops. This doesn't happen much if a quality modern boiler is correctly installed. The shower installation manual would advise on whether your cold pressure needs to be regulated down or not, that's a pretty trivial thing to do in any case.
