Boris Johnson's Covid-19 Lockdown Broadcast was watched live by 27m people

"Boris Johnson’s address to the nation ordering Britons to stay at home due to the coronavirus pandemic was one of the most-watched broadcasts in UK television history, with more than 27 million Britons tuning in live to watch him announce the historic lockdown of the country.
This puts the pre-recorded broadcast in the top tier of most-watched British television programmes ever, up with the likes of the 1966 World Cup final and the funeral of Princess Diana, making clear the historic importance of the occasion as the country collectively watched the prime minister give the latest coronavirus guidance.
The overnight viewing figures do not include the millions of Britons who watched the broadcast on internet livestreams through news apps and websites, or those who watched it through catch-up services, meaning the real audience is likely to have been substantially higher."
Source:  The Guardian, 24th March 2020

Note - It was shown simultaneously on several major channels