Camellia saluenensis 'Trewithen Red' won first place in the species class at the Wisley-based show, with the garden picking up 15 awards from 30 classes, including three first places, four seconds and four thirds.
The plant has been dubbed a "myth and legend" plant by RHS Rhododendron, Camellia and Magnolia Group chairman and International Camellia Society UK registrar Andy Simons.
"Camellia saluenensis comes from the wild in whites and pinks but not red. However, the little known 'Trewithen Red' is thriving in the garden," said Simons.
"A nearly mythical dark red C. saluenensis was reportedly growing at Trewithen many years ago as an original seedling from seed collected by the plant collector George Forrest.
"It was reportedly lost during a severe winter so this plant and the plant growing in the garden must be related or, if not, are the same. Maybe this red form of saluenensis can be used to produce the dark red hybrids so many people want."
It is over half a century since the Cornish gardens have exhibited at a London show. Head Gardener Gary Long said: "Having travelled a long way from Cornwall to show at this competition, we were really pleased to be awarded so highly - especially with the 'Trewithen Red'."
He added: "Many of our competitors grow their blooms under glass but our plants are grown in open land - thanks to the Cornish climate. 'Trewithen Red' has been growing in the garden for years and will now be recognised as an award-winner."