There is no evidence available that proves a robust "cause and effect" between the current national lockdown, and the drop in cases. The counter-narrative is that the rate of growth in cases was already reducing throughout the UK before the second national lockdown started (easily independently verifiable using the governments own stats dashboard), and cases plateaued in the early stages of the lockdown - happening too early to be reasonably attributable to the effects of the lockdown starting to bite.
Instead of downvoting why not prove me wrong? My first sentence is as far as I know plain truth, and the second is just describing the popular counter-narrative, not making any claims as to its veracity. When the social ill-effects of a lockdown policy are as severe as they are, we should be able to have a healthy public debate about the balance of harms and under-pinning evidence.
