Great question!!
Wikipedia has an interesting entry for List of aircraft of the RAF which contains an alphabetical listing of most of the RAF aircraft. No doubt someone will have a trawl through for a definitive answer.
A few come to mind: Bell Airacobra; Manchester, Buckingham, Welkin, but I feel that probably it will be a type introduced at the end of WW1 where rapid decommissioning of aircraft and squadrons occurred. Maybe a Martinsyde?
Found a Parnall Heck of which only one appears to have served in RAF, does this count??
Come on, love to know the answer!!