After spending way to long on the web figuring out what detailing is I came across a few posts on how some of these guys carry on using their pressure washers (PW) during the ban. They use rain water from their guttering system to water-butts via a downpipe diverter. Then through a filter to their PW.
Depending on your PW throughput, I'm guessing 100Ls of water would get you around 15mins of spray time? More than enough time to pre-wash, foam and final wash, right?
I might try it but I think I'll have a long wait before I get 100L of rain water and probably end up using the tap...