Sorry but it's simply to put the rich in power to drive their new EV SUVs while limiting people with less money from driving their own car. People who have 4 kids: "sorry your Citroen is not enough. Buy yourself an ID Buzz we don't care."
(Also note that the average family in the UK appears to only have somewhere between 1.7 and 2.4 kids, depending on sources.)
That doesn't change the substance of your point, though. Very few people living in Z1 or Z2 run a car unless they're rich: parking is extremely difficult and public transport is so good that there are very few reasons to want one unless you're regularly leaving London.