Cambridge recently released a study estimating that 9% or so of the UK population already have antibodies. You only need to get to ~60% and (assuming it doesn't become endemic) you have herd immunity, it doesn't matter that the virus is here to stay.
We don't even know still if immunity would last for more than a couple of months, it seems that would be a short duration issue in any case, so the herd immunity plan has some loopholes on it.
There is a paper (not peer reviewed) that suggests that herd immunity starts occurring around 20% as long as mass gatherings remain are banned. If true life could go back to normal a lot quicker.
