I am being optimistic. Sorry if you disagree! There is a chance that the virus has already torn through the population and once a reliable enough test is widely available they can sample the population. I have quite a few friends in London who think they have had it (although could just be after a bit of attention!). If enough people have been affected the r-parameter will reduce below 1 and it should then die off.
That would be the best case scenario, but for a big part of the population to have been infected and not know it, the death rate would have to be way lower than we think. But there is data from populations where everyone got tested (like cruise ships) which puts a lower limit on the death rate. Let’s say only 1 in 1000 people dies from it (which is 10 times lower than current estimates).
There’s been 12,000 UK deaths (officially, maybe that’s undercounted but let’s say it’s ballpark). So that means 12 million total cases. But we’re a country of 70 million people so that’s only 20% of the population.
You can play around with assumptions and estimates, also of course some areas are more affected so might have a higher local rate, but it seems very unlikely that >50% of people have had it and we just don’t realise.
Of course this is all just a “guesstimate” and population sampling for antibodies is the way to find out for sure.
It’s also not even clear how much having it once will protect you from reinfection. Not all diseases make you immune for life after one infection.... so even if 80% of people have had it, it might turn out you can get it again 6 months later...