The UK and Sweden believed at the start of the pandemic that herd immunity would be acquired in a reasonable period of time without too many deaths.
Why they believed this and how many deaths were an acceptable number is impossible to say.
Sweden had the added problem that there was no legal precedent for telling the Swedes that they had to close bars, stay in their homes as much as possible etc., so legally they could not implement a lockdown. Finally when the death toll rose to steeply they forced a bill through parliament so they could lock the country down.
Most other countries in Europe enforced a lockdown from the start - how severe it was and who it affected varied from country to country, but the lockdowns do seem to have saved lives.
All politicians reviewing what actually happened will say they acted on scientific advice. Probably most of them did, but there have been instances of governenments not listening to the advice they were given.
Scientists haven't always agreed about what was necessary either.
Boris Johnson will doubtless say, he did what seemed best at the time, which may be true, although nothing suggests that the man is particularly truthful.
Unfortunately, he comes across as an incompetent oaf, leaving many of us wondering how he ever got to the top of a political party.