Inspector First, that 197 is not cubic metre. It is a rateable value, which was the assessed rentable value of the property when they did an assessment a long ago. This was the practice used until 1990 - before privatisation. The water companies still use this to charge a percentage of the RV.
Based on your input, I will be surprised if your bill does not go down. You can also ask what the standing charges are if you instal a water meter. I do not think they can tell you what your water consumption will be. You can ask your neighbours about the costs and their experience, if they have installed a water meter.
I do not think they will not have to dig anything to install a water meter. It is normally a simple job. They will open the mains box - normally just outside your property on your drive way or walk way to the rear garden. They will shut the water supply, install a meter and that’s it. You do not have to be around. This has been our experience. I would give them a call, if I were you, and ask all the questions.