We talk about vaccines generating herd immunity, so why is this different? Because this is not a vaccine. This is an actual pandemic that will make a very large number of people sick, and some of them will die. Even though the mortality rate is likely quite low, a small fraction of a very large number is still a large number. And the mortality rate will climb when the NHS is overwhelmed. This would be expected to happen, even if we make the generous assumption that the government were entirely successful in restricting the virus to the low-risk population, at the peak of the outbreak the numbers requiring critical care would be greater than the number of beds available. This is made worse by the fact that people who are badly ill tend to remain so for a long time, which increases the burden.