When a submarine might fire a surface-to-air missile

When would a Royal Navy submarine use a submarine-launched surface-to-air missile?

Only when already exposed or located, because shooting usually sacrifices stealth for a last chance to destroy the hunting aircraft.

Firing at a patrol aircraft reveals a submarine's presence and normally sacrifices the protection of remaining hidden. A surfaced boat leaving port has a stronger reason to use a man-portable missile if it cannot yet reach its dive point and an enemy aircraft has already seen it. Once detected in that situation, diving does not erase the aircraft's knowledge of its position. The Soviet bastion rationale accepts an even worse position: a boat that believes it has already been located may take a last chance to destroy its hunter and reduce the available patrol force. Early Soviet cruise-missile submarines that had to remain surfaced while guiding their weapons also had a limited need for air defense. These exposed or desperate situations give a submarine reasons to fire that a hidden boat would normally avoid.