PN-You are correct
Henessey's text states that the AOC Bomber Command could, if all efforts had failed to contact the PM of the day and his nuclear deputies, authorise the V Force to attack their targets.
Ironically, this was apparently a similar strategy to the 'Wing attack Plan R' in Strangelove, and was built in as a nuclear failsafe in the case of the 'bolt from the blue' attack where the Soviets may have tried to decapitate the political command and control structure with a targeted attack on London/Turnstile?? and other important political sites.
Interestingly, when the RN took over the deterrent, no such authority was passed to the C in C Fleet (or similar). As Hennessey states 'special communication arrangements apply' in the same eventuality-presumably the 'letters of last resort' that all SSBN's carry.