The numbers of pence per kilowatt hour for an EV does not change dependent on the comparison to a petrol or diesel car. If we are talking about the efficiency savings, we would expect greater savings for the EV compared to a petrol car doing 30 mpg versus the comparison to a diesel car. And 3 miles per kilowatt hour isn't that unreasonable, you are not going to get 4 except in specific circumstances.
I think it's obvious that the more miles per unit time, the quicker an EV will pay off. But then, one does need to charge at home so the other challenges (off road parking, long enough between use periods) makes a large difference. It does not surprise me that it works better for you with a works van and high mileage, where it can be classed as a business expense with BiK bonuses. But it doesn't help Joe bloggs who does 6k/year at a push.
I'd like to buy an EV. I expect my next car to be one, whether I like it or not, and I've set everything up including solar panels to wring out every efficiency I can. But with this tax system and fuel cost differences the sums don't add up for me, and most other people.