Tootfinder

There are a few sites using the UK government API for fuel prices, but I suspect, based on military choices, the crowdsourced "availability" reports may actually be the thing that's critical.
(Not sure why availability wasn't part of the API, given the chaos of September 2021.)
https://checkfuelprices.co.uk

Check Fuel Prices – Find the Cheapest Petrol & Diesel Near You
Live UK fuel prices from thousands of petrol stations including Shell, BP, Gulf, and more. Compare prices for E10, E5, B7, and SDV near you using our interactive map or postcode search.