Warhead transition and stockpile logistics

Why did Britain raise its nuclear-warhead ceiling, and is it changing the submarine or missile type?

The rise provides transition margin for new submarines and warheads; the D5 missile remains while Britain and the United States modernize warheads together.

A very small standing stockpile leaves little slack when old and new warheads must coexist during a transition. Each deployed ballistic-missile submarine also needs a consistent operational load rather than a mixture assembled from whatever remains available. The higher ceiling therefore provides logistical margin while both a new submarine class and a replacement warhead enter service. The D5 missile remains, while bilateral arrangements link the British and American warhead-modernization efforts. Decoys occupy part of the missile load, so the number of missile positions should not be read as the number of live warheads carried.