Plenty of rental car companies offer unlimited mileage (especially in the US, Avis in particular).

I spent nearly a decade as a consultant traveling extensively throughout the US and Europe--and unlimited mileage rentals could be had for a reasonable and often surprisingly cheap price. I never understood why people opted to pay by the mile.

I mean exponentially as in the experience is radically different. VW, BMW, etc will disengage multiple times during a trip by itself. Autopilot does not. 99% of my trips, I do not have to take over from autopilot. It just works. Sharp curves, stop and go traffic. The intelligence of the system - Tesla will look at cars in adjacent lanes, detect a blinker and slow the car to allow a merge. Tesla will look at cars approaching from an on-ramp merge and slow down to properly zipper merge. Tesla will detect a car in a blind spot during a lane change and automatically steer to avoid a collision. And yes the automatic lane changes are even further. They don't require perfect road conditions and there are no issues with proper speed or accurate changes. For me in SoCal traffic, the car will drive onramp to offramp, changing highways and avoiding slow traffic without me doing anything. This is what I mean by exponentially different. It is night and day.

One small tip for renting cars is to look around at a few rental places, ideally in lower-traffic or lower-population areas. £300 for a weekend seems pretty high (but comparable to e.g. renting from the local airport here), whereas I could take an Uber to an Enterprise just outside of town for ~$30 and get a "premium" car for $140 for the weekend (48 hours at $70/day).

If you actually use the cat like that twice a month for a big part of the year it of course makes sense to buy one. But most people use the car for that fun drive maybe twice a year (even though they of course dream about doing it all the time when they buy their expensive car).