The governments consultation document can be found here
https://assets.publishing.service.gov.uk/media/69282ac1a245b0985f034197/eVED_Consultation.pdf
it says that you estimate your mileage at the start of the year, pay for that and if it’s more at the end of the year you pay the balance. If you’ve overpaid it’s carried over to the next year. Mileage would be checked at service and at MOT.
It specifically says that mobility cars are NOT exempt from this tax as they were not exempt from fuel duty but will continue to be exempt from VED.
It also mentions PHEV and says it would be too complicated to say what mileage was done on electric or petrol so they will charge 1.5p instead!
Note its only a consultation document so some things could change !!