Even that assumes that UK will be able to handle 2 million infected people at the same time. They can't.
We saw Italy's system collapsing with less than 100k infected people. We are seeing what is happening in New York with less than 100k infected people. It is hard to see a country the size of UK being able to deal with 2m infected people at all time, for 18 months. Heck, doctors and nurses will be already dead if they will have to work in a very infected environment, in double shifts for 18 months. It was always, a totally nonsense plan that had no basis in reality.
And as you said, fof that to happen, you need a uniform distribution. Which would be pretty much impossible to achieve. What is going to be instead is something that looks like a Gaussian, when 10m+ people will get infected in the same time, and hundreds of thousands if not more are gonna die within a few weeks.
Which begs the question, if countries have to do that after the lockdown, why bother with a lockdown in the first place? We actually saw University College's study that showed that a lockdown followed by going back to life as normal has the same number of infections and fatalities as no lockdown at all. Which is why the removal of constraints is going to be gradual, and life as usual will continue only in 2022 if we are lucky to have a highly effective vaccine. Until then, hopefully a South Korea scenario, which while is not ideal, it is pretty acceptable.