My own view is that this - if it comes at all - will not be implemented by cameras or tolbooths or black boxes on board cars. All of those kinds of solutions will be enormously expensive and costly and slow to implement.
The government will want something simple and easy to collect - so I think relying on insurance companies who already have to know your annual mileage, will be the way to do it, perhaps backed up by MOT data from year 3 onwards, which might also be used to beef-up insurance checks.
Clocking is already illegal and using the MOT data alone would unfairly target second hand car buyers.
This is simple enough - they wouldn't need to talk to the average car owner at all.
Failing this, they might require MOTs every year from new instead.
Yes, anything like this will incentivise clocking, so they may look at stronger safeguards there such as connected vehicles auto-reporting overall mileage, which could progressively come in as the fleet renews.