OK, yet another example of a media report that doesn't know what it's talking about.
"Software virus"? That's redundant. (I am willing to withdraw my objection when somebody shows me a hardware virus.)
Given the (minimal) information provided in this report, I strongly suspect ransomware.
Poor chaps...Tesco averages one major incident a year...
This article on Darkreading.com mentions that it might have involved exploitation of a vulnerability in Pulse Secure --- used for VPNs --- being exploited.
(The vulnerability was announced last year)
An Oracle WebLogic vulnerability that was patched in April 2019 was used. Sounds like RansomWare has taken it toll.