A mild hybrid has so little electrical assist they cannot run on electric alone, unlike a normal hybrid.
Please define a "Normal Hybrid"
I am led to believe the Pay Per Mile (PPM) will only apply to vehicles that can recharge their battery from a mains supply, in other words BEV or HPEV , so it excludes self charging hybrids, (Normal Hybrids?) which makes sense as self charging systems rely on power generated directlt from the use of fossil fuel or the motion of the vehicle which is only produced by fossil fuel used in the engine. They pay their fuel duty on all the energy they use to move the vehicle.
BEV's of course only use energy derived from mains supplies which have no fuel duty applied to teh tariff.
PHEV's are a compromise. They generally only have a small EV range only, and the rest of the energy is derived from a kinetic energy recovery through braking, and of course mains charging.
The PPM is clever approach, as for cost conscious PHEV users it encourages the driver to minimise the amount of petrol/diesel they use to avoid paying the fuel duty, by maximising the use of the battery.
Where this is less than ideal is for people who went for an early PHEV with a small battery and a very limited range, yet they have to do long journeys where the ICE has to provide the bulk of the power. The owner ends up paying Fuel duty, but also the PHEV PPM.