At this time, details of how the government plans to carry out this taxation are scarce. Maybe they don't know themselves.
The issues I see with this are:
1) Apparently it is to be a flat rate per mile, irrespective of car size. I drive a 6-seat Tesla MX, my mother drives a 2-seat Smart. For her to pay the same rate as me seem rather unfair.
2) If I take my car abroad, for example a summer trip to the south of France, I will pay for French motorways by the kilometers and I do not think I should also also be paying the British government for that distance. There has been no mention of this by the government, but there does exist a general principle against double taxation.
It seems likely to me that the government has not worked out any details and is not going to do so until very close to the time when this goes live. This to avoid giving citizens any opportunity to protest and/or mount legal challenges against unfair features of the way it is to done.