Do you know what the exact price was the day before, the exact price the next day and the price move in the wholesale market? Maybe they did preempt prices slightly - tis irrelevant and likely was a short lived increase in margin (if there even was one). If you consider that you're basing that on your own selective memory and are reliant on a guesstimate as to how much of it was just profiteering on the the part of the retailers then it isn't reliable. FWIW markets are generally quite efficient and whole sale prices move rapidly. The extra margin to be made is likely to be confined to fuel they already own and on the other side they can potentially also take a hit on their margins if/when prices drop suddenly....
You can just look at the figures and see quite clearly that it is a low margin business.
www.ukpia.com/files/pdf/ukpia-briefing-paper-understanding-pump-price.pdfA margin of 6% doesn't give much room for prices to be moved around by the retailer. Sure a supermarket can take a hit and discount by a few pence - for the independent owners, if the margins were any lower, then they'd might as well not even be running the business in the first place and just keep their cash in the bank/investment fund.
For people who complain about fuel - the main issue with it is the tax... for most of the population of the UK the place you fill up at doesn't have a monopoly and no one is ripping you off - its actually very fairly priced on the part of the supplier.