Have you tried the company that the site is linked to? Harrod Horticulture is still shipping some products.
https://www.harrodhorticultural.com/search.html?q=compostThe 6x chicken manure we got is £20 on amazon, which is £6 more expensive than a garden centre but one 20kg bag will fertilize 264 sq.m. 8Kg will do 110 sq meters or 132 square yards. It's pelleted for ease of use and limit mess. There's also smaller amounts as well if you don't need that much. You could add that to your dead compost or soil which is our plan. Amazon seems to be empting fast as well but there is compost.
If you don't want to go to the £1 shop why not book a taxi? Up here the taxi drivers are coming to your door, picking up your shopping list, buying your shopping and dropping it off at your door. Phone the shop and ask what is there so your not wasting money on a blind taxi run, then call a taxi and send them there for you. If there is a more out of town supermarket they maybe more likely to have items.
Have you tried a big gardening retailer or big supermarket that's out of the way? The problem is there are restrictions on essentials (Easter Eggs) and some shops will decide compost isn't an essential, it is up to their discretion so you'll have to move asap before they run out. We've got compost from both morrissons and Asda in the past 3 for £10 on big bags but they aren't getting more in-so I've been told.
Someone I know told me yesterday she'd been out to the most rural of supermarkets on the outskirts of the city because the big ones at the beach (Aldi and Asda) have had no meat for weeks. So they had to head to the out of town shop to stock up on everything as they'd emptied the house. But they got there at 6pm and there was loads of empty boxes so while they got something, it wasn't as much as they'd have liked. I don't know what time they went to the shop at the Beach, it could have been a case of her visiting after the morning rush when everything had been cleared but hadn't been topped up.
Modify Dobies online is that busy that they've had to shut down the site until 6am tomorrow. So yes, if your going for gardening website move asap in case they sell out.