If a water company finds you are using water for commercial purposes, they will fit a meter (and charge you for them fitting it) and charge you more for the water than for domestic use.
It nearly happened to me at my last house but as we were in the process of moving house, which we did 2 months later, the water company (Southern Water) didn't have enough time to pursue it legally.
If it was me, I'd let them come round and have a look but remove all your RO stuff before they arrive and say to them that you are a trad window cleaner which is what I should have done. As I found out to my horror, as soon as you deny them access, they assume you are 'commerial or business'.
Also, I would think the term 'sourcing water illegally' means using domestic water for commercial purposes.