A comment elsewhere pointed out that many people repeat the Excel story, but there are no primary sources.
Upd: BBC also updates on this subject[3] now, the facts will come later ("Investigation launched into coronavirus test error").
[1] https://www.telegraph.co.uk/technology/2020/10/05/excel-erro...
[2] https://twitter.com/standupmaths/status/1313055411285774336
[3] https://www.bbc.com/news/uk-54422505
Upd: BBC also updates on this subject[3] now, the facts will come later ("Investigation launched into coronavirus test error").
[1] https://www.telegraph.co.uk/technology/2020/10/05/excel-erro...
[2] https://twitter.com/standupmaths/status/1313055411285774336
[3] https://www.bbc.com/news/uk-54422505