A billion pounds in extra food in three weeks to a nation of nearly 70 million people, is a fiver a week a head.
At the same time that cafes, restaurants etc first empty then close, so less food being consumed there.
So really not a big increase. Some people will have spent more than that, a lot will have spent less, maybe even have decreased the amount of food they buy due to other financial pressures (e.g. job loss), or an inability to either go to the shops due to self isolation or find sufficient food when there.
It’s not panic buying, it’s that Just In Time cannot cope. And we’ve been directed to get angry with the moral failings of selfish greedy panic buyers rather than look at the failures of the system.
It’s difficult to know when and where a crisis will strike exactly. It’s not difficult to predict that there will be a crisis. Indeed, pandemic experts have been warning that something big has been overdue for a while. Food system is totally unable to cope with a few days of really bad weather, never mind a global pandemic.