As it happens, we watched another episode of The Prisoner this weekend, The Girl Who Was Death.
Patrick McGoohan, dressed as Sherlock Holmes, follows the girl of the title to a fairground. We reviewed the opening footage of this scene repeteadly; the set looks suspiciously like La Ronde.
The scenes were allegedly shot on location at the now defunct Battersea Funfair in 1967.
However, one shot includes, off in the distance, a flag that looks suspiciously Canadian.
J-F theorizes that stock footage of amusement park rides in action may have been used. I'm not convinced this makes sense if filming was to occur at Battersea anyway, but I have no alternate explanation.
If anyone can supply a reason for a Canadian flag to be flying at Battersea Funfair, or identify the flag that might be mistaken for Canadian, I'd love to hear about it.