Some lunatic will take it as a challenge :-) (and become a statistic)

Absolutely, doing it in half that time would be madness though!

I did see a Reddit thing where some tourists were planning to stay in the Lake District and visit Edinburgh and Stonehenge, all during winter.

Could've been ragebait, to be fair - they weren't interested when people pointed out that things like weather, hours of daylight, travel time were all going to be against them (or even that the Lake District is a pretty tourist-friendly place to start with).

Google Maps says between 5 and 6 hours and 227 miles - doing that in 3.5 hours would be averaging 65mph. Good luck with that, especially when the speed limit on the A9 itself is 60 mph for cars!

The US interstate is probably more comparable with UK motorways.

Other than a time lag (and petrol prices don't generally change often enough to matter IME), I can't say I ever noticed any of them being that inaccurate.

myAutomate (the owners of Petrolprices.com) talk about having "over 60 years combined expertise in the fuel industry", so I suppose I'd be surprised if it's all crowdsourced data - they've probably made arrangements with at least the big players, in which case the forced publication is much of a muchness?