Gordon's correct.
A quote from official drought order in the link says you can't use water for:
the cleaning of windows by hosepipe, sprinkler or other similar apparatus which is connected directly to the mains supply;
So since the majority of WFP systems aren't directly connected to the mains supply; it doesn't affect your work.
If you use a WFP and while working your system isn't directly connected to a mains supply; it doesn't affect your work.
I think the wording is worded to prevent people from watering their lawns, and when caught, claim they were cleaning their windows.
I'm sure 'they' don't want to prevent you from making an honest living; they're only after stopping garden-proud home-owners from keeping their grass wet; and providing them with a 'get out of jail' excuse.
So, after reading the drought order; if your WFP system isn't DIRECTLY CONNECTED TO THE MAINS SUPPLY, then you can still work.
I'm no lawyer, but it seems simple enough to me.