Drag, in 1963 that was the policy decision for clearances.
My source also includes Harrier, Jaguar, Lynx, SHAR, Tornado and Vulcan Mk 1.
On the Vulcan re-roling to WE177 was simply a matter of swapping panel EP for panel ER. I know however that the Mk 1a Vulcan wing at Waddo was never roled for WE177 - targets or training.
I have photographs of a Wessex carrying a WE177A test round and a later list had plans for Wasp and Wessex to be cleared in 1967 and Sea King and Nimrod in 1968. I know Nimrod never carried WE177 either.
Although the Scimitar and Sea Vixen had been on the list they were both removed and the F4 was not added.
What I don't have is a definitive list of what actually carried the WE177. I know the Jaguar did but the Harrier did not.