On
14 May Minutes of Sage meeting contains the first mention of asymptomatic individuals and transmission estimating 10% - 35% truely asymptomatic (point 32).
On
7 July BBC reported
"Only 22% of people testing positive for coronavirus reported having symptoms on the day of their test, according to the Office for National Statistics.
This hammers home the importance of "asymptomatic transmission" - spread of the virus by people who aren't aware they're carrying it."
Clearly there was an awful lot that was not known about the virus at the time, on 10 March for example SAGE thought there was maybe 5,000 - 10,000 cases across the country.
And when you read the minutes from the time SAGE was very worried about the surge overwhelming the NHS so the policy is understandable.
There's also the point that the last place an elderly person wants to be during an epidemic is in hospital surrounded by patients with the disease so it was also in their medical interests to be discharged.
So really the government was f***ed either way, discharge them and risk spreading the virus, or keep them in hospital, sentance them to death and run the risk of overwhelming the NHS.