Mmmm.
Not exactly. The real catalyst for going from an air launched deterrent was that the USA 'pulled the rug' on Skybolt, leaving us with a bunch of unpalateable alternatives - continuing with Blue Steel, which was no longer viable, developing a new air launched weapon in an impossibly short timescale, or doing as the US wanted and adopting Polaris.
There was also the perceived importance of the Moscow criteria, under which it was felt to be essential that the UK deterrent should be able to threaten Moscow itself, in the face of an over-exaggerated (largely perceived) ABM defence shield. This criteria later drove Chevaline.
While air bases are inherently more vulnerable (in certain respects) than submarines the advantages and disadvantages are far from clear cut, and it is apparent that the USAF felt that with the right alert posture and deployment capabilities, an air launched deterrent remained entirely viable throughout the Cold War period.
Today, moreover, the Moscow Criteria are entirely irrelevant, and we are not looking at needing to be able to 'launch the deterrent' in the face of some kind of Armageddon-like mass strike by the might of the Soviet ICBM/SLBM/ALCM forces.
Trident (and any sub-launched follow on) represents a massively expensive, relatively inflexible 'overkill', when a cheaper, less capable air launched nuclear option might be better suited to Britain's post Cold War defence needs and budget. Nuclear armed Storm Shadows (say) carried by Tornado, FOAS and MRA 4 would not be remotely 'good enough' if we wanted to penetrate the ABM defences we thought existed around Moscow during the Cold War, but in a post Cold War environment would be good enough against even the most difficult real world targets..... and therefore pretty credible, I'd say?