I built this using the official UK government forecourt fuel price feed.

The map aggregates the latest petrol and diesel prices by county, with filters for fuel type and metric. Clicking a county shows the cheapest forecourt, average price, spread, and station count. The feed covers roughly 8,000 UK forecourts and refreshes every 30 minutes. Retailers publish the prices, so there can still be gaps in the data/stations but it's getting better over time.