It is staff shortages compounded also in cases by a shortage of truck drivers:
this from CNN:
" ...Another major supermarket chain, Sainsbury's (JSAIY), said it was "working hard to ensure customers can find what they need." A spokesperson for grocery chain Co-op said it was "running low on some products" but "working closely with our suppliers" to restock shelves.
The rising virus caseload — and subsequent increase in the number of isolating workers — is also exacerbating a shortage of truck drivers caused in part by Britain's exit from the European Union.
Worker shortage is forcing UK businesses to close as Covid cases spike
Rod McKenzie, managing director of policy and public affairs at the Road Haulage Association (RHA), said the United Kingdom is facing a shortage of around 100,000 truck drivers, 20,000 of whom are EU nationals that left the country after Brexit. He said that increased red tape at border crossings following Brexit was also putting European drivers off making trips into the United Kingdom.
"If the 'pingdemic' hadn't happened we'd still be in trouble," he told CNN Business."