The balloon flight was organised to celebrate the coronation of King Edward VII, and was piloted by M. Auguste Eugine Gaudron and Dr Francis Alexander Barton. It ascended from the Croydon Road Recreation Ground in Beckenham, and mail was dropped at three places in Kent, before the balloon finally landed at Calais. The photograph shows an aerial view of the recreation ground taken in the 1900s, although the balloon in the picture is not, as far as we know, the actual one involved in the flight, any more than the one carrying the camera.
The auctioneers Warwick and Warwick describe how post was sent using these very early balloon flights: "To celebrate the coronation of Edward VII, in August 1902, a balloon flight was organised at Beckenham in Kent. Three bundles of 100 postcards were flown and I believe only two have survived. Then, a month later, a balloon flight was planned to raise funds for the Lifeboat Service. The public were invited to purchase specially printed postcards depicting a hot air balloon bearing the banner Lifeboat Saturday."
That page has photos of the Lifeboat postcards (but sadly no photos of the Beckenham ones). The Lifeboat balloon "landed at Ongar in Essex, from where the postcards were forwarded to their addressees.... They both received the Ongar c.d.s. before being sent back to Manchester", Manchester being the delivery address on the cards. (In philately circles c.d.s. stands for 'circular date stamp').
The plaque on the shop at Thornton's Corner declares that was the first British airmail post office and, since we can find no reference to the shop being an actual post office, we've been thinking what that might mean. Seems likely that the flight organisers wanted to raise some funds to offset the cost of the flight. The public interest in balloon flights was enormous so they came up with the idea of selling postcards that would travel by balloon before entering the standard postal system.
The postcards needed to be designed, printed, stocked, advertised and sold. The flight organisers probably would have had no experience of any of this but locally there was the Thornton firm who could do all that. There may also have been franks or stamps - imitating those used by the Royal Mail, and making more of an event out of buying the postcard.
We not claiming that the flight organisers invented this fund-raising model. Looking around for other examples we found PretmoiParis: "... there is a post office on top of the Eiffel Tower. Send your post cards from there! It’s fun and they get a special ink stamp on them when they are sent." Linns and the Bulletin of the Civil Service Philatelic Society give references to this happening in 1889 and in 1936.
Credit for this entry to: Alan Patient of www.plaquesoflondon.co.uk