My Holy Grail was found last week - Military Orchids, appropriately in the grounds of a castle, Chateau Gaillard on the chalk cliffs above the north bank of the Seine in Normandy.
Since the castle was built by Richard the Lionheart to defend his (i.e. English) lands from the French, this clearly is a British tick.
The name derives from the "helmet" formed by the hood which is clearly different from the otherwise quite similar Monkey Orchid.
An area within a couple of hundred yards of the car park also held Fly, Lizard and Greater Butterfly Orchids and White Helleborines. A sign by the local conservation society effectively reads "the flora here is threatened by tourists - please tread carefully".