It is a decision last faced by a British government 25 years ago. In 1980, Margaret Thatcher signed off on the shape of the nuclear deterrent that would last into the 2020s.

The decision put British-made nuclear warheads atop US Trident missiles to be carried by four nuclear-powered submarines to be designed and built in the UK. The first Vanguard class submarine, each of which could carry up to 16 D4 missiles with a 6,000-mile range, was commissioned in 1993; the last in 1999.