Filmmaker Samantha Horley recently posted an image of this set of "Guidelines," which she found among her father's effects, on her Facebook page. Horley told me that her aunt worked at the BBC as a secretary in the 1960s and 1970s; she thinks the page originally came from her aunt's papers.
The BBC's press office told me, over email, that the page looks like it came from The BBC Variety Programmes Policy Guide For Writers and Producers, published in 1948. Although the BBC spokesperson couldn't confirm this theory, I think this sheet was probably printed up for the amusement of employees in the more free-and-easy 1970s.
The BBC reprinted the entire document as a book in the late 1990s; it's now out of print, but here is a version in PDF. The longer document includes provisions that are less overtly amusing than this section but are interesting nonetheless, offering guidelines on libel and slander, religious and political references, and jokes about physical and mental disability.
Under the heading "American Material and 'Americanisms,'" the anonymous authors of the handbook observed that "American idiom and slang" were often found in scripts and that "dance band singers for the most part elect to adopt pseudo American accents." This "spurious Americanisation" should be avoided, the handbook urged, since it was "unwelcome to the great majority of listeners and ... seldom complimentary to the Americans."