Thanks, Paul. So, does this mean that the two possible systems are: 1) the traditional gravity system; or 2) a system whereby it's connected directly into the mains?
Basically, yes, as far as your shower installation is concerned, although there are a number of variations in the detail and you can find hybrid systems, e.g. the tank in the attic feeds just the hot-water system while all the cold taps throughout the house are direct from the high-pressure mains supply.
Sometimes there is a combination hot-water cylinder & cold-water header tank for the hot water while all the cold taps come directly from the supply. That arrangement is quite common in apartments and other homes without sufficient roofspace, where the combination unit can be located in an airing cupboard or similar place.
Systems employing the modern "combi boiler" usually provide both cold & hot water at mains supply pressure.
The main issue as far as a shower with a conventional mixer arrangement is concerned is that both hot & cold feeds to it are of near-enough the same pressure. So to cover the possibilities:
1. Hot & cold from a traditional gravity fed system. Hot & cold pressures will be equal, but may be insufficient to give a shower with some "punch" behind it. This is where a booster pump will be beneficial.
2. Hot & cold through a "combi" unit, or where cold is otherwise direct from the supply and the hot system is pressurized. Hot & cold pressures will be equal, and should be high enough to drive a conventional mixer shower adequately.
3. Cold direct from the supply, hot from a gravity-fed system. Not suitable to feed a mixer shower tap directly due to the vastly different pressures. Unless you're going to replace the hot-water system, options are either to run a new cold feed to the shower from the header tank, if possible, or to use a pressure balancing valve. The latter can work only by reducing the pressure of the cold water to that of the hot, so you might end up with the same low-flow problem as with the full traditional system, but this time you can't get around it easily with a booster pump.
I don't know how to explain this, but Tim's shower is turned on from a switch, and then it has dials that control the water temperature.
This sounds a bit like Steve's shower. In his house you pull a cord in the bathroom to switch on the shower and the ventilation for it, and you use the dials on the actual shower unit to control the temperature and all that loveliness.
These are almost certainly instantaneous electric showers, which are the other option. They require only a single cold water feed, and heat the water as it passes through. The cold supply needs to be at high pressure (direct from the supply usually), and the temperature is set by adjusting the flow rate (i.e. increase the flow for cooler, decrease the flow for warmer).