This is exactly 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.