Ask yourself this. If lockdown is a law and not just an unenforceable rule/guideline, then why was parliament asked to debate and vote on it? If it is a law, then parliament would not be allowed to debate or vote on it since the PM would have the legal power to implement lockdowns without requiring parliamentary consent to do so.
And, if the country really is in the middle of a serious health crisis and the evidence was unassailable, there would also be no need for a debate or vote. Lockdowns would just go ahead without any challenge from MPs or the public.
Fact is, lockdowns are not lawful. They are simply rules you do not have to obey. And there is absolutely no unassailable scientific evidence that proves we are in the middle of a serious health crisis either.
Lockdown rules are not law, there is no health crisis and that is why Johnson needed to convince the rebel MPs to vote in favour of tiered lockdowns.