They need £x per year for y usage of the aircraft. Whether they charge a higher rate for wheels up to wheels down, or a lower rate for engine start up to engine shutdown, or an intermediate rate for brakes off to brakes on, is up to them, as is whether they calculate taxy time by flight time plus ten minutes, or flight time by block time minutes ten minutes, or actually time taxy time by watch, or whatever.
For the same aircraft and the same level of service it'll even out, you'll pay the same amount for the same flying regardless of the charging algorithms.