Anti-Brexit campaigner Steve Bray played the track 'Yakety Sax' near Parliament as people were waiting for the Prime Minister's official announcement.
He apparently did so after a request from Grant, who tweeted: "Do you by any chance have the Benny Hill music to hand?"
Bray later shared a video showing him playing the track loudly.
"Just for @HackedOffHugh as requested here today at the media circus… College Green," he wrote. "The Benny Hill theme tune."
— Steve Bray Activist Against Brexit +Corrupt Tories (@snb19692) July 7, 2022
Journalists covering developments nearby pressed on, as the jaunty music threatened to drown out their words.
Viewers thought it was absolutely hilarious.
"Hahahaha this is the best thing I’ve seen this week," one person tweeted, while another wrote: "It was glorious, I couldn't stop laughing!"
"Loving the circus music heard in the background now as the UK networks broadcast outside Number 10," said another person, adding that it was "fitting".
"This is absolutely incredible," tweeted one person.
Another said watching "Chris Philp MP on College Green being accompanied involuntarily by the Benny Hill theme tune while commenting on the unfolding fiasco is just perfect".
"When people ask about Britishness this is the sort of thing that I think quantifies it," posted another person. "The ability to make the serious hilarious."
Johnson announced his departure around lunchtime on Thursday, saying in his speech how sad he was to give up “the best job in the world" but that "them’s the breaks".
The Benny Hill Show - which starred comedian and actor Benny Hill and featured slapstick sketches - aired from 1955 to 1989.
Watch: Hugh Grant called 'Jesus' for donations to charity