This footage was found on a reel of 16mm cine film amongst in the RBGE film collection. This one was a reel showing George V and Queen Mary with our Regius Keeper at the time, Sir William Wright Smith.
The group are filmed walking through the Rock Garden and adjoining Heath Garden which was established in 1935, quite possibly for the Jubilee. Queen Mary was known to be a keen gardener, and apparently the royal couple made informal visits to the Garden throughout the '20s and early '30s.