This is a brilliant article. It confirms the government's move away from "herd immunity" was not based on changed modelling. They really had planned to allow 250,000 people to die, and were taken by surprise when the public weren't willing to accept that. https://www.reuters.com/article/us-health-coronavirus-britain-path-speci-idUSKBN21P1VF

What changed was the behavioural science. The government had assumed we would prefer to let 250,000+ people die rather than spend a few weeks at home.

There was no real effort to ramp up testing or increase number of ventilators. It looks like the government only sprung into action once they saw other countries were behaving differently and felt pressured to change.

Look back to the 12th March press conference and its clear: Johnson wasn't levelling with us that our lives would change, or that the state would need to enter a war-style economy to increase capacity. He was getting us used to the idea our loved ones, 250k+ of them, would die.

I should clarify as a couple people have brought it up. The article doesn't confirm 250,000 always predicted to die, but that re-thinking plausibility of lockdown was main driver of change, not science changing. @ShippersUnbound suggests original figure was 100,000.

If we look at the statements of Vallance and Whitty around the 12th March Press Conference, we can see 100,000 definitely wasn't considered a maximum. They repeatedly said 60-80% population would get infected, death rate unknown, but close to 1%. That's a lot of people!

Be it 100k or 250k, the takeaway is that the government assumed the public would accept mass death, and only had to change course when neighbouring countries took a different course, and when a political backlash ensued.