Every region in which I've held a licence, including the UK, defined 'time in service' (TIS) ie accrued time for the purposes of maintenance, as '...from when the aircraft became airborne until it next alighted'. Synonymous with 'wheels off to wheels on'. That period is the minimum amount of time that must be recorded to determine maintenance interval. Time accrued that does not involve flight is not considered for TIS calculation.
As long as whatever method you use accrues *at least* that time then it doesn't matter (to the Authorities) what method you use. They don't care if required maintenance is completed more frequently. However, the owner/operator should! Doing maintenance more frequently than is required makes the operating cost of the aircraft greater.
At a minimum recording time by the clock meets the rules. Tacho fudges the period because it is not strictly recording that which is required. It is arguable that, at continuous low power flight, the operator is not complying with the regulations. I know I've done numerous flights where the tacho showed less time accrued than the TIS. If tacho is used to track TIS in those cases then the rules are not being complied with.
I don't know why many operators don't make use of weight-on-wheels switches - when available - or fit an airswitch to activate a Hobbs recorder. It makes determining TIS sooo much easier.
Charging for the aircraft use is up to the operator. Any method that the users will bear is fair game, although using the same Hobbs that tracks TIS makes things easy. It also removes the price pressure of the user to rush warming & conducting pre-takeoff checks. If renting 'wet' then adding an allowance for fuel used during start & taxi takes care of the - slight - additional cost from that part of the plane's use. Admittedly the catch is that it adds motivation to run the engine hard, but a price penalty based on tacho difference soon stops that behaviour, as do excess fuel consumption surcharges.