(1) Why can't UK build its own functional equivalent of Trident? Keep British jobs at home for the lads, and so on.
(2) Why not build submarines similar to the Virginia class, which have vertical launch tubes for either Tomahawk cruise missiles, antiship missiles, or larger torpedos?
(2.a) I'd suggest building a ballistic missile small enough to fit in those tubes. This missile might carry either nuclear or non-noo-klur warheads.
The basic idea is to have multi-role submarines, instead of specialized, use-only-on-Doomsday ballistic missile boats.