Not sure how widely available it is, but I get cashback from Tesco via Airtime Rewards (for links, see here: Airtime Rewards). I didn’t realise it includes fuel too, but it was a nice little bonus.
It is ridiculously expensive at the moment though. Our local Tesco has lagged behind in prices (thankfully) and only recently increased to 145.9p, previously 140.9p for unleaded. The next valley over is 150.9+ 