I was looking at this and I thought it was a pretty confused article - it talks about UK, 80% and 500,000 and then has a chart of the fatality rate in China - if you combine the age profile of the UK with that fatality rate and times 80% you get double the 500k mentioned in the article.
https://www.theguardian.com/world/2020/mar/13/how-many-will-die-of-coronavirus-in-the-uk-a-closer-look-at-the-numbers
I was looking at this and I thought it was a pretty confused article - it talks about UK, 80% and 500,000 and then has a chart of the fatality rate in China - if you combine the age profile of the UK with that fatality rate and times 80% you get double the 500k mentioned in the article.