Yes, I have Anglian Water for my sewage service.
Following my phone conversation with them as per their instructions: I fitted a water meter to the supply to my RO unit in the garage and give them a reading every 6 months - 6 April and 5 Oct - so they can deduct an amount for water not returned to drain.
Technically the meter should be fitted to the product water pipe from the RO or have one fitted in your van to measure the amount of pure used - but I won't tell them if you won't. (I reason that waste water from the RO should need little in the way of treatment in their sewage works, that's what helps me sleep at night anyway).