First BBC Broadcast (14/11/1922)

It was the British Broadcasting _Company_ at this time, owned by a consortium of wireless companies, who feared the "chaos" attendant on the expansion of radio broadcasting in the USA at the time.
Business-wise it did not work out very well, and in 1927 it was taken over by the government and became a "Corporation", with a public-service "charter".
"Its first operating license restricted broadcasting to news and information from just four news agencies. Daily broadcasts began in Marconi's London studio, 2LO...in the Strand. A news bulletin went out at 5:33 p.m., along with a weather report, spoken by the Director of Programmes, Arthur Burrows, in an authoritative RP accent."
Burrows also played Father Christmas in _The Truth About Father Christmas_, thought to be the first broadcast drama. And he was the first "Uncle Arthur" on _The Children's Hour_.
So the RP accent became known as "BBC English". The Advisory Committee on Spoken English was set up in 1926 to provide approved pronunciations for new words and foreign names, and as an authority to support news readers against the inevitable complaints. A fascinating body in which both Daniel Jones and George Bernard Shaw were involved.
My impression is that it was not until the 1970s that a wider range of accents began to be heard.
https://en.wikipedia.org/wiki/BBC