Basic premise seem to be that you can save around £1,000 a year by switching from a petrol/diesel to an electric car.
My basic maths suggests that you won't get payback for 20 years or more. That is if you buy the very cheapest model, costing somewhere above £20k. It looks as though anything half decent will cost more than £30k.
Of course, the car will have to last that long, or have a good residual to cash in.
It suppose it makes sense if you were going to buy a new car anyway, and decide to buy an equivalent electric car for up to £5k more so you get your extra money back in 5 years.
However as a basic choice of swinging in your old but still working oil burner for a shiny new electric thing it doesn't seem to make economic sense.
Cheers
Dave R