A pressure washer generates a very high pressure at the nozzle (hence its name) but the flow rate is quite low (only about 6 litres a minute). This flow rate is probably lower than the mains water flow rate out of the tap you are attaching it to. The pressure on the water main is, therefore, not going to be reduced by the pressure washer.
To get a decent shower (like the one I've just had), you are going to want at least 12 litres per minute, preferably more. If this is more than the water main can supply at its normal pressure, then the pump will reduce the pressure behind it and could lead to reduced (or no performance) at other outlets.
That's why you are allowed to attach a pressure washer to you garden tap but you are not allowed to attach a shower pump to your rising main.