The UK had a perfect storm of factors contributing to the severity of the Covid pandemic in that country: highly internationally mobile population, highly internally mobile population, a global travel hub, high population density, culture of close social mixing, no culture of mask wearing, colder climate leading to more indoor living, higher than average levels of elderly people, obese people, and of the ethnic groups most impacted by the virus, low appetite for strict enforcement of lockdowns, and low levels of government control over everyday living.
Other countries had some of these factors. Few if any had all of these factors.
What happened next? The vaccines turned out to be really effective.
The UK certainly doesn't have all of them for a start.
The lock-downs had high levels of compliance, just a lot of people moaning from their sofa (or whatever soapbox they have available, reach varies greatly, inversely with intelligence in many cases).
Population density, travel hub, obesity levels, delayed first lock-down would be my list.
Other countries had some of these factors. Few if any had all of these factors.
What happened next? The vaccines turned out to be really effective.