The environment would be better off if I just kept my current petrol car.

It’s a fair point, it might be true in your personal circumstances but the answer will depend on the age, pollution levels, miles driven, value and the cost to fill up your existing car, this will be down to you doing the maths comparison, regardless “In all the cases examined, the electric car has a climate advantage over the combustion engine over its entire life cycle.” [source], so when looking at buying a replacement car moving to a fully electric or even a plug-in hybrid should provide you with much lower running costs, lower longterm environmental impact and most importantly less local pollution. You would also be better off and so would the environment if you sold your existing car and moved to walking, cycling and public transport but considering a large portion of the UK population commute to work via car it’s unlikely that will be convenient for the majority of commuters.