Low Pressure Gravity System
This is probably one of the most common systems in the UK, which is slowly being replaced with newer systems. It usually consists of a cold water tank in your loft, and then a further heating tank located somewhere else on the premises (most often in the airing cupboard). The higher that the tank is from the endpoint (i.e. your shower or tap) the more pressure that you will see. A very rough calculation is 0.1 bar for every meter above the endpoint the cold water tank is. So if the cold water tank in your loft is 3 meters above the kitchen tap, then you will see roughly 0.3 bar pressure. Obviously this can be lower, depending on the quality and age of the pipework that the water is running through. Older systems may see less pressure due to build up inside of the pipe over time.
If you have this system you can install a mixer, pumped digital or all-in-one power shower and you will have get ideal performance from these products. If you decide on a something with a higher pressure requirement due to design limitations on low pressure taps, fitting a pump will boost the flow rate & pressure even more.