No, it wasn't. But there was the small matter of a national lockdown, that almost certainly made a difference.
I can't prove a causation between the two, but it defies common sense to argue that the March lockdown had no impact on the spread of the disease. In effect, you seem to be arguing that the success of lockdown is an argument against it.
Nor is it true or fair to suggest that SAGE scientists "don't have any comprehension" of the economic and other costs of this. Of course they do. When Professor Whitty was pointing out that there are "only bad options", this is part of what he meant. The choice appears to be this: let many thousands die (and, in the process, overwhelm the NHS and wreck the economy), or try to take action that will reduce the chance that the NHS is overwhelmed, and in so doing wreck the economy.
It's no answer, either, to point to the Nightingales going unused. They were a contingency measure. It's a success story that the contingency turned out to be unnecessary, not a failure.