Wouldn't it be cheaper to extend the life of Trident and of the
Vanguard boats?
On the subject of Trident, you might be interested in these two papers....
New roles for VANGUARD
and
Modifying capacity
What about combining a V/STOL type aircaft with nuclear tipped stand off weapons? The number of places such an aircraft could operate from (no need for a long runway like conventional aircraft) must be high. This mobility would give it great survivability, with them being spead over a wide area, with the flaxability that missiles don't give you.
Or am I being dumb?