When we lived in High wycombe (Thames Water) we changed to a meter in early 2000's and it took about 7 years until we were paying as much as the old system (by which time the price of the old method would have gone up even more). The next door neighbours changed to a meter and their charges went up a lot (even though there were only 2 of them). There was a leak somewhere but fortunately they were on top of it, and opted for the old system before the period of grace ran out (I think TW agreed to charge them as per the old system).
So the moral is, if you do change, keep well on top of new v old before you're stuck with it.