There are many check valves in home.
Cold feed into your home
It is a legal requirement that you have a double check valve on the mains pipe coming into your home. This check valve is to prevent back flow of water if there is a burst water main and there is a legal requirement to have a double check valve. If you suffer from low pressure mains pressure and you have the normal 15mm double check valve then replace it with a 22mm check valve and use 3 part 22mm to 15mm reducers.
There is a 22mm single check valve within the body of a ShowerPowerBooster but water will bypass the check valve in either direction because of the unique patented way we pump water. If you have need to prevent water flowing back from a mains pressure cold because of a failed check valve in your system you can fit a 22mm single check valve either before or after the showerPowerBooster.
Mixer Showers and Taps
There is normally a non return valve which is 15mm or less on the hot side and cold side of a mixer valve and this valve is part of the mixer. There is no law which prevents suppliers selling substandard mixers into the UK which do not have non return valves and many of the mixers sold, including a few well know prestigious brands, have non return valves with springs that prevent water at low pressure from forcing its way past the non return valve. If the flow to your tap or shower is not sufficient to activate the SPB flow switch then removing the non return valve on hot side of the tap could solve it. If you have mains pressure on the cold and gravity hot then you will need a 22mm single check valve before or after the ShowerPowerBooster