Yes, Costco sell diesel for 3p per litre less than the local garages. For me, it's a 5 mile round trip to the nearest cheap garage, a 46 mile round trip to the nearest CostCo filling station.
I cancelled my CostCo membership because it cost more than it saved me, though that was before they sold fuel.
I doubt I'd ever get the £26.40 per year back. After allowing an extra 2p for the "premium" added to the fuel, that 5p saving means I'd have to buy 528 litres to break even.
If you include the extra distance to fill up, it never pays off: assuming I put in 75 litres each time, that would mean I'd save £3.75 per fill. Driving 40 miles costs more than £3.75.