Just checked some lease costs out of interest:
BMW 330d Msport - cheapest lease I can find is £25,413.84 for 48 months (
https://www.cars2buy.co.uk/personal-car-leasing/deal/2fe211017eb871a985c0ef28c27a3520b176d3bc)
(does not include many of the options which are standard on the Tesla but ignore that)
Tesla Model 3 SR+ - cheapest lease I can find is £25,314.16 for 48 months (
https://www.cars2buy.co.uk/personal-car-leasing/deal/6337360500ae073e7912a9240573b84074a11099)
I was going to suggest the fuel saving would cover the difference but the 330d is actually a bit more expensive to start off with. Over 40,000 miles the Tesla would save you about £4000 in fuel (if you can charge mainly at home). You'd also pay nothing for servicing/brakes etc.. so that's at least another £500-£1000 depending on usage.
So in the region of £5k saved over 4 years, not to mention the evnrionmental benefits of saving many tonnes of CO2.
This isn't a mega-scientific example but I think it shows that the fossil fuel option isn't by default cheaper if you're buying new.
If buying nearly new with a personal loan (as I have historically done) then it needs another year or two for more used electric options.
Note: I'd probably try to buy the Tesla outright rather than pay £25k to rent one for 4 years as I think it's residual would be pretty good - but won't go into that here