Not sure how your school at Biggin will charge, but at the school where I do most of my instructing just down the road at Redhill we charge for airborne time (i.e. take off to landing, measured by a meter that switches on above 30Kts) plus 0.2 hours taxy time (12 minutes). This is a good system because there is no incentive for the students to taxy too fast or skip the warm up. You won't end up paying more just because ATC keep you at the hold for a long time either.
The time you can log is from when you actually move off from parking up to when you get back to parking after the flight (or stop at the fuel bowser). This time is often more that the time charged for, especially during the early lessons when students take a long time to do their power checks.