In thinking about why the UK was impact worse than others, I basically I think there's three broad categories to consider. Firstly, is what you might call the basic make up of a country - population density, age profile, number of ports, lifestyle habits, population attitudes to authority, stuff like that. If the UK is shown to be more susceptible due to, for example, an older population or more dense cities, then clearly Johnson can't be held responsible for that.
Secondly is the quality and quantity of its relevant infrastructure - the NHS and social care providers notably, but also factoring in the capacity of Whitehall, Public Health England, its Local Authorities, the third sector, and so on. Shrinking these structures has been the concerted aim of the Tories for a decade now, so if the state of these structures turns out to be a meaningful factor, then the Tories as a whole take responsibility for that.
Finally there's the handling of the crisis from the day news arrived on our shores - how quickly we locked down, how quickly testing came on line, how information and advice was communicated, the quality of the Government's communications, etc. Clearly this is 100% on Johnson and his team.
In the final reckoning it will be impossible to quantify exactly which of these factors was the most important. They all exacerbate each other after all. However I think that when you look at the issues that have cropped up, the diminished state of our public infrastructure and the uneven and frankly wonky response by Government have both been telling components.