Edgley Optica - could it ever have been better than a heli for observation?
Large turboprop civil transports - The order books for the Britannia, Vanguard and Electra did not justify their development, albeit of course the Britannia and Electra were splendid airliners. The planform has not emerged again. On the other side of the wall, I accept the Il-18 was built in good numbers, but only 32 Tu-114's emerged from the shed.
To take a slight left turn from PDR1's Concorde line, maybe we can add "fast civil transports", the original failed attempt to establish this brand being the CV880 and CV990. Post SSC, Boeings subsequent paper Sonic Cruiser ended up in the waste paper basket