Well my 20 year old car meets the various clean air zone emission standards that newer cars fail to

However even if it didn’t, if I used it for 200 miles a year would it make sense to buy a new electric car?

It’s never clear cut, and it’s practically impossible to make the best decision in any given case. You can make a regulation which will on aggregate lead to less damage but there will always be exceptions, and on a case by case basis it’s extremely difficult to measure the damage a given scenario applies. How many “units of badness” does buying a new 2 ton electric car before you move it a single mile. Id wager it’s more than an existing petrol car burning 1 litre of unleaded petrol on existing tyres and brake discs.

The difficulty is measuring total impact of the choice. Sure buying a new petrol car and driving 20k miles a year for 6 years will be worse than buying a new electric car and driving 20k miles a year for 6 years. That’s not where the line is.