On the back of this portrait I found pasted two small photographs of London. Trafalgar Square (top) and Carlton House Terrace on The Mall. Union flags everywhere and Nelson's column is wrapped in a garland. Several Flickr detectives have been poring over these for me trying to establish a date and identify the occasion. The vantage point for both appears to be Admiralty Arch (completed 1912). A sign for the Liverpool & London & Globe Insurance Co Ltd can be seen. They moved to that address in 1919.
Finally, someone came up with this postcard. The obelisks visible on The Mall were erected specially for Peace Day in 1919. I think that nails it. Thank you, everybody!
See some colour photos of the event here.