So from looking through their external services, it looks like M&S started containment between Sunday 20th-Tuesday 22nd of April.
They had inbound network activity from IPs associated with crimeware groups, but difficult to know which one due to shared infrastructure.
The Sunday Times today back up my toot about VPN access to M&S being shut down, saying a source at the company says remote access has been “scaled back”: https://www.thetimes.com/business-money/companies/article/marks-and-spencer-shuts-out-wfh-staff-after-cyberattack-q2wwcjzl3
Additionally, the print version of The Telegraph says it is ransomware, citing somebody involved in containment. They say they believe they have locked the threat actor out of the network now and are working on restoration.
Marks and Spencer cyber incident = ransomware.
It's DragonForce ransomware cartel, they have encrypted their VMware ESXi clusters. Lines up with network traffic I saw.
Ongoing outages at British retail giant Marks & Spencer are caused by a ransomware attack believed to be conducted by a hacking collective known as "Scattered Spider" BleepingComputer has learned from multiple sources.