I know someone who avoids their local petrol station that is 10p/litre cheaper than most others nearby (within a mile or so) as they think the cheaper fuel must be lower quality. There are weird status things going on with purchases like this.
Only the other day my father refused to buy some branded paracetamol because it was ~5 times more expensive than the local pharmacy brand that was out of stock. (£2.25 vs £0.49 for 16 500mg tablets.) I'd usually agree with him but he was out of paracetamol and has been advised by his doctor to take 2x500mg a day and there was no viable nearby alternative.
A digression but for that generation (those born in 1940s/50s) that grew up with rationing I think it is hardwired into their brain to try and minimise the cost of so many things, but with lots of random exceptions. Later on that day he ordered an extra drink but decided he was too full once it had arrived so he left it. So he was worried about spending an extra £1.76 on paracetamol but not about spending £7 on a pint he didn't drink.
Many people decide what petrol station to use based on simply how close it is, what kind of shop is attached to it (and the bits of British snobbery around that), whether it also sells whatever else they want (bread, milk, beer, etc), or even whether it is easy to drive in and out of.
I don't know if your experience is from British people but it looks like they just didn't have the mean to effectively compare fuel prices.
Once they do, there is a significant part of the British drivers that will most likely be using it.
We have a system here in Western Australia and people use it a lot: fuelwatch.wa.gov.au
I think it's exactly that, the UK has never had this so people there either choose by brand or just convenience. But since moving to WA I've found that it's really easy to have a quick look when I notice I need to fill up, then I can head to the cheapest station nearby, and the difference can be in the range of 10-15%, occasionally 20%.
In a country where fuel is as expensive as it is in the UK, people are going to use that.
We've had it for years (as noted in other comments there's a few different people like the RAC, AA and Petrolprices.com all maintaining their own lists - a quick check of my email has messages from the latter going back to 2011). The new part is that this is from the government and the data is freely accessible (Petrolprices in particular covered their pages in ads, so I'd be surprised if there wasn't a way to exchange money for the data).
The context to this is that, especially since the pandemic, there's been a complaint with the Competition and Markets Authority that the petrol stations were quick to raise prices, slow to lower them, and weren't competing with each other[1]:
> The CMA found that retail prices tended to "rise like a rocket, but fall like a feather" in response to increases or decreases in the cost of crude oil.
Independent petrol stations have virtually disappeared and you don't have to look too hard to see that in an area they tend to all raise or lower their prices in virtual lockstep. Gathering this data would make the case significantly easier if the next step were that some of the petrol station operators had to be broken up to encourage more competition.
1: https://www.bbc.co.uk/news/articles/cp80dpzdg37o#comments
Edit: Petrolprices was founded in 2005 (!) [2]
Petrolprices.com (for example) seems to have been built on user-reported data rather than petrol-station reported data, and it's easy to find fairly recent criticisms of the whole thing being inaccurate. And an inaccurate comparison site is fairly useless IMHO.
I lived in the UK until 2021 and I must admit I'd never heard of them. Whereas here in WA everyone uses fuelprices. There are probably other factors involved here as well, as we have a weird weekly or biweekly price cycle (though I think this has ended somewhat in the last two years) where every second Tuesday fuel was dirt cheap as they were trying to clear down the tanks ahead of the next delivery.
Is the 'new part' not that the vendors are being forced to actually publish comparison data rather than rely on third parties to gather it?
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?