Yes, but my very point is that 1st March was already too late, but we didn't know it.
Half term was the previous week (or 2 as there is a bit of spread of weeks), where people had gone into very infected regions without knowing that they were, and they brought it back here, where it spread in the community as it had done there, without their knowledge 2 or 3 weeks earlier.
Oh and 80000 went to the rugby so why single boris out ? He didn't know, just like the other 79,999 people, that lots were already infected, and were infecting others, particularly in the southeast, where people share a transport system for work, where you can pretty much breathe straight up someone elses nose.
Were mistakes made ? Yes, and they still are being, but this is worldwide, and as you keep saying, this is a new virus, so we still know relatively little about it, but it's a whole lot more than the nothing we knew about it in early March.