Problem with collecting rainwater is a hose-pipe ban is likely to last months, literally, even once its started raining a hose-pipe continues to be enforced.
If a window cleaner uses lets say 300 litres a day, he works 22 days a month, and the ban lasts 3 months, then hes going to need to store 20,000 litres for that period of time, thats 100 water butts, thats £3,000.00 in water butts alone if theyre £30.00 each.
Or a bloody big pond.
Its not impossible, but if thats the route your going down I suggest you start digging today.
Matt