The earliest Outside Broadcast I remember was when I was a boy shortly after the war ended. Children's Hour (radio) broadcast a radio commentary from an RAF Stirling bomber flying over London.
I lived in North London at the time and followed the broadcast intently watching out for the Stirling as the commentator reported what part of London he was flying over. In due course it appeared flying over Broadcasting House and firing off Very Lights, very visible to me and my schoolfriends in my house as we listened to the broadcast.
I guess it took place in 1945/46.