My OP was really about the fact that the statement made it appear that Boris Johnson doesn't understand the effect of the lockdown on the timing of the peak of the epidemic. (This is an objective fact and is not difficult to grasp - it is obvious to anyone who thinks about it).
Another thing that was obvious from the start was that if applying no control measures, we would probably end up with around half a million deaths. You don't need a complicated model for that - just an estimate of the death rate (1%) and the population size (60 million) and the proportion who might be expected to catch the virus (80%). This is why is is false to claim that the imperial college study was what changed the governments minds - that piece of information was nothing new. What the study gave us what some estimate of the different effects of applying various lock down measures. Really though, as anyone who's done any modelling will happily tell you, models are usually wrong to some extent, and this one in particular made loads of assumptions that are not backed up by science - simply because the virus was new and we did not have access to the relevant facts at that point.
There are lots of different factors that might affect the decision on when to impose the lockdown, what measures to take and when to release them, which need to be debated at length by those in charge. There aren't any easy answers.
I guess one thing that has begun to worry me is that by delaying lock down and allowing more spread of the virus throughout the population, it made lockdown disproportionately less effective for the some of the most vulnerable than it would have been if it had been applied say 2 weeks earlier.
This is because many vulnerable people can't lockdown effectively. Those in care homes, hospices, hospitals, those with regular carers visiting their homes, those needing help from family with daily activities, those needing to attend hospitals or GPs for regular treatments. These people continue to be as risk even though the rest of us are locked down, because a later lockdown meant that even though these vulnerable people probably hadn't been to the pub/school/cheltenham races themselves recently, some of their carers probably had.
Furthermore, because the young and healthy are now mostly isolated, they are not getting infected and therefore are not contributing to herd immunity. So it is possible that lockdown applied quite late actually makes the most vulnerable more at risk - because they are the ones who cannot isolate and therefore are bearing the brunt of the spread of infection.