If such a signal ever goes out, then the Submarine Parachute Assistance Group (SPAG) deploys.
Part of the Royal Navy, the SPAG specializes in the recovery of submariners. There are several jobs within the SPAG, such as medics, engineers, and submarine escape specialists—but all members are proficient in first aid, escape-and-recovery techniques and submarine, surface-to-air and ship-to-shore communications.
SPAG teams are on constant alert. In the event that a Royal Navy or foreign submarine requires their expertise, they can be underway in less than 6 hours.
Once parachuted, either from a helicopter or plane, depending on the incident’s proximity to the shore, they loiter above the stressed submarine in rigid-hulled inflatable boats. For additional muscle, they also operate rigid-hulled inflatable life-rafts, able to hold 25 people. All of their boats are furnished with communication and GPS devices. The life-rafts, moreover, store hot and cold rations, oxygen therapy devices, and more specialized first-aid equipment.
They can also deploy from surface warships.
The SPAG was created in 1967 amidst the Cold War. With the British and NATO operating numerous conventional and nuclear submarines, the Royal Navy figured out that the probability of an emergency was high, and thus came the SPAG.
The SPAG is commanded by a Warrant Officer 1st Class and members of all services can join the unit, though most tend to come from the Royal Navy and the Royal Marines. Parachute qualification is a requirement for acceptance into the unit.
The SPAG was first deployed in a real-world operation three years ago when the Argentinian submarine ARA San Juan went missing — despite international efforts to locate the submarine and rescue the crew, the ARA San Juan went down with its full complement. The closest that they had come to a real operation before that was when the HMCS Chicoutimi, a Canadian submarine, caught fire near the Irish coast in 2004. Before they could respond, the Canadian crew had managed to address the situation and the SPAG team was stood-down.