It sounds like the UK hasn't had its Chicken Tax[1] moment in history yet.
A Chevy Suburban seats up to 9 and has a curb weight of ~6,000 lbs on the upper end.
A Tesla Model 3 seats up to 5 and has a curb weight of ~3,800 lbs on the lower end.
Take a wild guess which vehicle your proposition taxes favorably...something about unintended market consequences comes to mind.