This is a Charles Skilton postcard in a series of twelve which were published in 1950 or 1951. This is Speakers or Orators Corner in what I think is summer 1950, Cumberland Gate can just be seen on the extreme right with the Marble Arch behind. The orator is speaking on behalf of 'The Society for Evangelizing London', it appears to have been a one man band run by a Rev. J. Alston Campbell who may well be the orator shown, he was a Scottish baptist minister who had been a missionary in Turkey at the turn of the century. He wrote a book about his experiences entitled "In the shadow of the Crescent".
I have found only one advertisement and it is from the personal column of The Times of 5th October 1938, in which he asks readers to "Send your peace thanks-offerings to The Society for Evangelizing London which proclaims the Gospel and assists needy cases". He goes on to say that "Interviews by appointment for those seeking spiritual help".
The peace thanks/offerings refer to the appeasement by Neville Chamberlain when he signed the Munich agreement with Hitler and Mussolini on 30th September that year and thus avoiding war for the time being. If anyone knows more about the Reverend, please let me know.