I always thought the advantage of a sub-surface system was the alility to be anywhere off of the UK almost completely undetectable, preventing the enemy from taking our system out so they can use their system? The subs have had a continuous 50 year patrol and at anytime there is at least one of the four (three yet?) patrolling UK waters.
Surely this type of continued detterance and secrecy is less achievable by air means?