It's behind a paywall so I can't link it, and a long article, but this is the section from which the quote above is taken (Sunday Times) ...
A shock to the system
The meeting that will change British society for a generation took place on the evening of Thursday, March 12. That was when the strategic advisory group of experts (Sage in Whitehall parlance), the government’s committee of scientists and medics, gathered to examine modelling from experts at Imperial College London and other institutions.
The results were shattering. A week earlier, councils had been warned to expect about 100,000 deaths from Covid-19. Now Chris Whitty, the chief medical officer, and Sir Patrick Vallance, the chief scientific adviser, realised the estimates were wrong.
“Unmitigated, the death number was 510,000,” a senior figure said. “Mitigated we were told it was going to be 250,000. Once you see a figure of take no further action and a quarter of a million people die, the question you ask is, ‘What action?’” Another insider said: “There was a collision between the science and reality.”
Ministers had been on notice that drastic action might be needed since the virus first emerged in China’s Wuhan province in December. In January, Whitty told the cabinet: “It either stays in China or it will get everywhere.” For two months the government had time to prepare, but Johnson’s instincts were to resist a life-changing crackdown. “There was a lot of talk about how this was just a bit of flu,” one senior Tory recalled.
Dominic Cummings, the prime minister’s senior aide, became convinced that Britain would be better able to resist a lethal second wave of the disease next winter if Whitty’s prediction that 60% to 80% of the population became infected was right and the UK developed “herd immunity”.
At a private engagement at the end of February, Cummings outlined the government’s strategy. Those present say it was “herd immunity, protect the economy and if that means some pensioners die, too bad”.
At the Sage meeting on March 12, a moment now dubbed the “Domoscene conversion”, Cummings changed his mind. In this “penny-drop moment”, he realised he had helped set a course for catastrophe. Until this point, the rise in British infections had been below the European average. Now they were above it and on course to emulate Italy, where the picture was bleak. A minister said: “Seeing what was happening in Italy was the galvanising force across government.”
By Friday, March 13, Cummings had become the most outspoken advocate of a tough crackdown. “Dominic himself had a conversion,” a senior Tory said. “He’s gone from ‘herd immunity and let the old people die’, to ‘let’s shut down the country and the economy.’”
Cummings had a “meeting of minds” with Matt Hancock, the health secretary, who wanted stronger action to prevent NHS hospitals being swamped. Department of Health officials had impressed on Hancock that the death rate in Wuhan province was 3.4% when the hospitals were overrun and 0.7% elsewhere in China.
Johnson had also been queasy about the previous original approach. “Boris hated the language of ‘herd immunity’ because it implied that it was OK for people to die,” a senior source said. “Matt hated the language because it implied we had given up. You’ve got to fight.”
Cabinet ministers say Johnson has been far more decisive at crunch moments than his predecessor
Herd immunity
The problem for the government was that at the moment herd immunity was being banished from policy, it had become the focus of publicity. That Wednesday, David Halpern of the Whitehall “nudge unit” put the phrase in the public domain. Two days later, Vallance repeated the idea on Radio 4. With Italy, France and Spain going into lockdown, the government’s critics accused Johnson of refusing to act because he wanted people to get ill.
Insiders say it was “very bumpy” that Friday. “The meetings were very messy,” said one source. But when Johnson gathered his key advisers in the cabinet room at 9.15am last Saturday there was unanimity. Whitty and Vallance explained that Britain had been four weeks behind Italy “and now we are closer”.
The two experts, together with Hancock and Cummings, all delivered to Johnson one message: “Now is the moment to act.” The prime minister agreed: “We must work around the clock and take all necessary measures.” One of those present said: “The mood in the room was astonishing. You could tell that something very significant had shifted.”
Flesh was added to the bones in another crunch meeting in Downing Street on Sunday night and again in the 9.15am meetings and bilaterals between Johnson and key cabinet ministers throughout last week.
The media was briefed that elderly and vulnerable people might have to self-isolate for a period of months and that everyone else would have to engage in “social distancing” — working from home, avoiding groups and unnecessary outings. Most significantly, without a gargantuan package of support for businesses, renters and the self-employed, millions of jobs would be lost and the economy would collapse.