[figaroniguzo]

It's not hard to pick these things apart. Infections peaked before lockdown (in both lockdowns) in the UK thus lockdowns weren't necessary.

People decided to be vigilant by themselves — sure. That still means that lockdowns weren't necessary.

[rjknight]

That is one interpretation of the facts. An alternative one might go something like this:

In early Autumn it was apparent that infection rates were high in the North of England but lower in the (wealthier and economically more productive) South. The UK government adopted a local/regional approach to lockdowns, with the effect that the North was under lockdown-like conditions for most of Autumn. Infection rates began to fall in the North, but simultaneously started rising in the (non-lockdowned) South, though from a lower base. Once infections started rising in the South, the government adopted a national lockdown plan (with, coincidentally, vastly more financial support for firms and individuals). And so the fall in infections we see "before the lockdown" is the effect of the "tier 3" restrictions across the North, with the national lockdown intended to prevent serious increases in the spread of the disease across the South.

I am not saying that this story is true, just that it's an equally good fit for the facts. Presenting statistics is only ever the start of the conversation, not the end, as the statistics do not speak for themselves.

[alkonaut]

The conclusion doesn't follow. Perhaps the lockdown was the reason people were vigilant? If so, then the mere news of the lockdown made people vigilant and aware of the severity of the situation. It would make sense that peoples' behavior changed early then. And unless one could have conveyed the severity to the population in any other way - the lockdown was "Necessary" even if just as a means of signalling.