100% on the ball. We sell quite a few pressure washers (Wickes branded, but made by Lavor) and the manufacturers recommend having decent pressure from the tap. Although, even with this in mind, it still amazes me when a customer tries to return one that "doesn't work" only to be told they switched it on before even connecting the hose from the tap - yeah, because you've burnt out the pump
A few manufacturers, either via leaflets in the box or websites, sell pumps or suction hoses to use with water butts/ponds to ensure an adequate supply of water to the internal pump for this type of supply