http://www.thameswater.co.uk/UK/regi...FAQ_000051.jsp

Washing flags is not explicitly banned, so you wouldn't be prosecuted.
It's a question of conscience.

David W.E. Roberts wrote:
Hi,

cross posted to gardening and DIY as probably relevant to both :-)

My plans to power wash the flags at the back of my Berkshire property have
been thwarted by the hosepipe ban.

As far as I can tell, power washers don't use that much water.

So is it feasible to put a pump in a rain water butt to feed water to the
power washer?

Has anyone tried this?

TIA

Dave R