When I started WFP four years ago, I wrote to Southern Water (e-mail) and told them what I was doing.
The only reply I got was concerned with the possibility of a ban on the use in the event of a drought order. No mention of wanting me to go onto a meter.
A couple of years later I contacted them again as my usage had gone up (a lot). I had a discussion with someone in their office who tried to say I ought to go on a commercial metered system. I asked if they could force me to - they said no so I didn't :

There are at least four separate records of me contacting them regarding WFP, but never any insistance that I should go on a meter.
Instead of worrying and guessing, you should contact them and tell them what you are doing. That way they can't turn round in the future and start making silly demands for backdated usage etc.
Even if you have to go on a meter, it's just a business expense, and in the great scheme of things only a minor one.
There are far more important things to spend your time on, like becoming a millionaire before you retire
