Sorry but that is incorrect regarding servicing as some manufacturers still don't keep cross dealer records. A large percentage of leasing is fleets of under 5 vehicles and will not have maintenance agreements and there is no need to report servicing (some just write their own). Add this to the number of people who take a lease or PCP on 8k per annum and don’t think there’s anything wrong with John from Facebook adjusting the miles every year to keep payments down.
The sad thing is clocking is very common across lots of types of vehicle contract, I have known hire companies vehicles be adjusted on long term contracts to avoid going over monthly mileage limits as well as privately owned and leased vehicles
The only way to stamp it out would be to make mileage adjustment illegal and have large fines for anyone involved.