Yes, this is true, but I wasn't meaning total immunity. I was meaning a reduced rate of infection, which starts to become apparent at a much lower level. There seems little down that many hundreds of thousands of people in London will have been infected?
That's surely a result of herd immunity? Also worth noting that the % of the population required for herd immunity will be higher in more densely populated areas, and the measures needed to reduce R.
In essence it's bloody complicated which is why it has to be done on a national level.