I just spoke to someone in the UK with direct interactions with senior government officials in a medico-legal capacity, and their assessment is that the "herd immunity" decision is absolutely not based on science, but rather a grandiose notion of the PM dressed up for presentation by his Chief Scientific Adviser and others as being somehow medically sensible. They estimate that there are perhaps 10,000 cases already in the UK, and the government's approach is an attempt to jump ahead of that failure, in effect saying, "Well, of course, we meant for that to happen. It's all going according to plan, you see?" The person I spoke to has 35+ years of experience in dealing with governmental foolishness, and was truly on the edge of tears with anger and frustration "for the first time in 20 years".