People can be wrong. If we're to believe this not to be the case, then they need to be more transparent, certainly with other experts.
ATM they are effectively just telling us what will happen 'when the plan works'.
But, for example, as a WHO spokesperson pointed out this morning that the Herd Immunisation strategy wholly depended on individuals generating the required immune responses to this particular virus. And she was lost for any evidence or data at this stage that actually supports the belief that this IS what happens with THIS virus. Which leaves us with the delicate question of wether they are going with the assumption that because it's worked that way with SOME corona viruses, it absolutely will do so in this case.
They most definitely are either (a) wrong, or (b) being dishonest. Let me explain why:
You cannot transfer immunity for any virus from one person to another. You get immunity by either catching it, or by being vaccinated. That's assuming you can get immunity from this by catching it and there is even a question mark over that. But let's assume that you can.
For herd immunity to work, it is estimated that 60% of the herd - the UK public - need to have immunity. i.e. 60% of the people in the UK need to catch it for this to work. You need 60% so that every time a new person is infected, they go on to inflect less than 1 other person, and then the virus naturally dies out. At the moment, each infected person is infecting between 2 and 3 other people, which is why the numbers are exploding exponentially. So 60% of the UK population need to catch it, some to die and some to recover. Until then, it continues to spread virulently.
60% of the UK population circa 40m people. Even if we were to spread this infection out evenly over 2 years, it's still nearly 2m people per month. The NHS cannot cope with anything like that. Not even close. And that's assuming no peaks, which is ludicrous. Of course there will be a peak.
So the herd immunity line is a complete lie. It's technically possible, provided we simply accept hundreds of thousands of deaths as an unavoidable outcome. A calculation on a spreadsheet might suggest it is a possible course of action. In the real world, with thousands upon thousands of lives at stake, it is a complete non-starter.