I don't think it's totally the case that the opposition viewpoint doesn't matter. At least not in terms of understanding why certain decisions were made.
It's often the case, and often really the ideal, that a ruling party will consider the standpoint of the opposition when formulating policy. Especially where it's not something that is a bill but just decision making in the face of a crises.
The fact is that the zeitgeist at the time was very pro-lockdown. And everyone was unsure about how serious it was, and maybe more importantly initially many people thought it would be short-lived.
No political party or PM has a huge appetite for doing something that the public don't want, that isn't what other countries are doing, and which the opposition is arguing against.
It was notable at the time the degree to which MN, and the media, and even medical experts, were wanting BJ to do more. Enough that he abandoned his original plan. There really was a kind of mass hysteria.
To the extent that Starmer was pushing that same narrative, rather than asking pointy questions about economics and civil liberties, he has some responsibility.