In the UK there is usually a significant price range from motorway service stations (high, but most people who use them are probably paying with company expense account) and the big supermarket chains (low, with more discounts on the pump price if you use stor loyalty cards etc). I just checked a UK fuel price search website. Within 5 miles of my home the range is 144.9p/liter down to 132.7 (which is the local Asda/Walmart price, actually 131.7 if you pay with their store credit card). That range is about 70 cents a US gallon.
The national fuel chains tend to match each other's prices to within 1p a liter (5 or 6 cents a US gallon) within a few miles distance. On the other hand in rural areas the "only petrol in town" is often at the top of the price range.
I've never heard any horror stories about "cheap discount fuel" in the UK. Nobody is going to make much profit trying to undercut the "big four" supermarket chains, and they wouldn't want to annoy millions of customers nationwide.