PG, I would guess that an aircraft deterrent would enable a visible show of force that could be both launched and recalled with the submarine as a secure backup and second strike system.
Before the RN deterrent the V-Force could be launched but if it was recalled the entire deterrent would have been shot for between 4 and 6 hours while a smaller proportion of the first launch was recovered, turn round and recocked. The Blue Steel systems might have been down for 3 days or more.