The gravity systems in this case are not pressurized. They just have a water tank in the loft/airing cupboard and the hight of the tank determines the pressure. 0.1 bar for every 1 meter height. You open the faucet and gravity pushes out the water.

Its a nightmare, I used to live in UK and these systems are barely enough for anything really.

These pressures are all gauge pressure, not absolute pressure. 1 bar gauge pressure would be about 2 bar absolute.

1 bar is enought to lift water 10 meters up. The pressure gauges reads zero at atmospheric pressure.