But not motorway range. Hyundai UK say:
If the manufacturer's quoted 'up to' range is 315 miles then the real-world motorway figure in winter will be a *lot* less than that. A quick Google found this test ... 153 miles at 70 mph took the battery down to 25%, the point where you'd be looking at needing a charging stop. And 195 miles to fully discharged (not recommended).
We charge a 2021 Hyundai Ioniq 5 to 100% and then drive it at a constant 70-mph to see how far it will go in the InsideEVs 70-mph range test.
insideevs.com
This may not be the exact same model but it gives some idea of the impact of motorway speeds on EV range. FWIW back home on Sunday night I had a remaining range of 254 miles shown, so let's call it 600 motorway miles on a tank of unleaded.
From another quick Google it looks like the above Ioniq 5 costs around £48k. My 2019 C300 (currently on 32k miles, so not a car I'd be expecting to sell anytime soon) is perhaps worth around £21k, so it would cost me £27k or so to change and start benefiting from the lower mileage costs. 10k miles a year currently costs me about £1,600. Let's say I only ever charged my new EV at home (best case scenario), then taking your 80% figure I'd be saving £1,280 a year. Assuming I had a spare £27k knocking about to start with that would be a payback period of 21 years ...