I think that stopping the production of high-emissions cars is as good a way as any - just phase them out. They will get fewer in number if people can't buy new ones. Maybe have incentives to scrap them or trade them in for electric ones (although as you say, Norah, electric cars are far from being the ideal); but penalising people for using something they bought in good faith isn't the way forward.
Who knows - by 2030 electric cars may be on the way to being phased out because of the ethical implications of lithium mining, difficulties in disposing of the batteries, their weight and impact on roads and car parks, and the reliance they will give us all on China. I'm very surprised there is no pressure group campaigning to get rid of them now. If that happens, what will people who have shelled out tens of thousands for one do? Again, if you are rich it won't matter much, but if the car represents a big chunk of your savings, you are struggling to pay for expensive food and are about to be hit with a huge bill for a heat pump or modern boiler, it could be the last straw.
People generally act in good faith, I think, and if the population can't trust the government to play fair by them there is instability. Although the UK is generally averse to civil unrest, people will only take so much, and we forget that we are ruled by consensus at our peril.