We had a significant reduction in social contact prior to the first lockdown which will have had an effect. We were also reaching the end of the normal flu season, so a seasonal disease such as Covid will naturally also decline. There was much better awareness of the symptoms and existence of the disease so symptomatic people were largely isolating. Sick people started avoiding hospitals, which are always huge spreaders of infection.The additional data we now have actually confirms fullfact's point about herd immunity: the major outbreak in December 2020 pretty conclusively demonstrates that we hadn't reached herd immunity and consequently that herd immunity cannot explain the reduction in cases we saw in the first lockdown.
Can I ask, do you believe the lockdowns were not responsible for the reductions in cases and deaths seen after their introductions? If so, what do you think caused these reductions?
Take your pick really - the point is that it isn't a binary choice between lockdown and doing absolutely nothing.
Herd immunity likely had an impact later on in lockdown and over the summer and autumn, reducing the speed of the second peak considerably, but it won't have had a lot of effect prior to or in the first couple of weeks of the first lockdown. I don't think anybody apart from you was really suggesting that it had.
There are some fairly reasonable hypotheses around immunity and the December peak. Obviously if you are approaching the immunity threshold, and a much more contagious variant such as the Kent one comes along and raises the threshold, then you are going to have a peak. In the case of the Kent one it also appears to have been able to reinfect some people, particularly those with weak or fading immunity, which again takes you further away from the herd threshold. It spread very aggressively however, so the natural herd immunity effects will equally have grown quickly.