The 2-man principle was intended to ensured that no 'unauthorised acts or incorrect procedures' were made when the crew was involved in nuclear weapon activity.
As
50+Ray states, it would be most likely that the weapon would have been released automatically. A sequence of checklist items involving the navigators and first pilot would ensure that the bomb would release when the NBS signal was given.
There was an 'instantaneous' manual release button on the centre console between the pilots and another on the co-pilot's side, which was routed via a time delay unit. Another manual release/TDU button was situated at the rear crew's station, as was the TDU. I guess there was another at the visual bombing position, but we never used that in my day.
V-force crews were subject to various security assessment checks; once the positive release code had been received there would have been no question of anyone being so disloyal as to try 'abort' nuclear weapon release.
The 'approved' visual bombing technique in my day included setting up the SFOM bomb sight and calculation of the correct TDU setting from the release initiation point. In practice it was simpler (and achieved better scores) if you tracked the target through the probe and pressed the centre console button just as you thought it was in line with the no. 3 rpm gauge.
We had a back-up dayglo band around the probe on one aircraft, it corresponded to the release parameters used for an earlier bomb comp. When asked at air shows why it was there, the usual reply was "It's just lipstick"...
