Although this doesn't apply to me, am I right in thinking that sewerage charges are partly assessed by the amount of water that you use, therefore the purified water that goes on the windows doesn't carry a sewerage charge? If so, I imagine that a secondary meter measuring what goes into the tank (AFTER the waste division) would be the answer.
As for them saying that water on windows goes down the drain anyway, that's only a partial truth. Some may go into storm water drainsd but much of it goes to ground or evaporates.
Another issue with this is that: Although your water is supplied at one point, the window cleaning takes place at multiple points. Make sure that Southern Watert are aware that most of ytour window cleaning takes place in other water company areas. My domestic supply is by Southern Water (not my work supply though) but my sewerage is by Thames Water. If you have a similar situation, is it possible that you should be speaking to Thames Water?