Why Western SSBNs avoid bastions

Could Britain and France assure their nuclear deterrents through a joint NATO submarine bastion in the Mediterranean?

Potentially, but it would require a major reversal of Western SSBN doctrine, which relies on hidden ocean patrols and protection against hostile tails.

Western nuclear doctrine generally sends ballistic-missile submarines into broad ocean patrol areas and relies on their ability to remain hidden. A joint Anglo-French Mediterranean bastion would reverse that established approach. Frigates with towed arrays, attack submarines, and maritime patrol aircraft help protect departures against hostile submarines trying to follow a deterrent boat. Seabed hydrophones are suggested as another possible layer, but their presence is explicitly conjectural. Russian attempts to trail British submarines are described as continuing after the Cold War, so protecting departures remains an existing requirement. The missiles' own guidance is distinguished from GPS-based position finding for the submarine.