Across Civil Defence Region 5 - which was the 28 metropolitan boroughs of London, most local authorities had area markings with their own name (e.g. Lewisham, Kingston, West Ham). The below photo shows the rarer "London" area marking. I'm unsure where this badge was issued, but possibly the outer boroughs may have issued the badge.
UPDATE: I have been informed that "LONDON" was an area marking used by the London Auxiliary Ambulance Service (LAAS).
News about interesting insignia, ARP related info and period photos that turn up.