Well, they argue that a couple it requires ICU capacity to be above 60 beds per million people. Britain's ICU capacity is on the borderline (~66), which may be a factor, and of course, social distancing wasn't particularly well adhered to, hence why lockdown was issued after the weekend when people appeared to go hiking in the national parks en masse.
Perhaps another factor is the annual vulnerability to flu. Countries like Italy and Spain suffer from seasonal flu, due in part to underlying health issues that render their populations vulnerable. I've said repeatedly on various threads that we shouldn't continue to live unhealthily and expect the NHS to pick up the tab. It'll come back to bite us, and it's noticeable that this is happening here during the pandemic.
The wife and I were debating this last night as it seems bewildering that Czech is seemingly doing well despite making so many bungles. One factor in their favour is that despite being heavy drinkers, the population is generally not overweight. Here, that's certainly not the case, and certainly not among many of the ethnic minority communities that have suffered most from the virus.