Let's stop panicking about hosepipe bans :
We had a hosepipe ban in the South East for two years on the trot. How many window cleaners did it affect?
A hose pipe is only a hose pipe if it's connected to the mains.
A WFP system is technically a "bowser and dowser". In other words a powered watering system attached to an independent reservoir of water. You have a tank of water in your van/trolley/backpack, you attach a pump, either electric or hand operated, and a means of directing the water where you want it - a watering wand, or a WFP.
Provided your pole isn't directly connected to the mains water, there's no problem.
There could be a bit of a problem if you have a van mounted RO and you run a hose pipe to your van to fill your holding tank via your RO.
Solution? Get a static purification system and storage IBC in your garden/shed or wherever, and then pump the pure water into a tank in your van. If your static system is plumbed into the mains, you can't be accused of using a hose pipe.
It's only when the situation becomes critical and the Government grants water companies Drought Orders that it could affect us, and then all water users are affected, not just window cleaners.
Cheers,
Ian