Trident

[True Federalist (진정한 연방 주의자)]

The Vanguard class carries 16 Trident II (D5) missiles with British made and controlled warheads.  Frankly, given that the U.S. has made plans to extend the service life of the Ohios and their D5 missiles, I suspect that the most cost effective way for the U.K. to maintain their nuclear force would be to do the same with the Vanguards.

Given the small number of warheads deployed, it would expensive to do what the French are doing and build a complete system from scratch.  Switching to the Le Triomphant/M51 system in place of Vanguard/D5 would be even more expensive as you'd either have to rebuild the Vanguards or build four new subs from scratch as the M51 and D5 missiles are not compatible (the French M51 is shorter and fatter than the D5).

The same imperatives that led the British to reject a land-based system in favor of Polaris in the1960's still apply.

Unless the U.S. decides to develop a new cruise missile program soon, I can't see either an air or sea launched nuclear cruise missile as being a viable alternative for the British either.  The AGM-129 production lines have been closed for over a decade now and reopening them for a British order of 100-200 ALCMs would be expensive and the very real desirability of keeping a distinction between nuclear and conventional weapons systems means that a nuclear tipped Tomahawk would not be a good idea.  Indeed, I doubt we would sell the Brits Tomahawks if we knew they were intended to tipped with nuclear warheads instead of the conventional ones that all current Tomahawks carry.

In short, from the POV of having an effective nuclear deterrent at the lowest possible cost, I can't see any option other than maintaining and upgrading  the existing Vanguard/D5 system as being practical for the U.K. for now.

[True Federalist (진정한 연방 주의자)]

Not replacing Trident wouldn't mean unilateral nuclear disarmament..

For Britain it would as it currently is Britain's only nuclear weapon platform.  At the very least Britain would need to develop a nuclear armed version of the Storm Shadow AGM for use by the Typhoon, but even with a stealthy aircraft and missile, I'd hate to have to be the pilot who had to get within 250km of the target to launch it.  Also, it is definitely better from the standpoint of avoiding accidental nuclear war to have distinct weapon systems for nuclear and conventional weapons.

[True Federalist (진정한 연방 주의자)]

From what I've read, you Brits weren't utilizing Trident D5 to its full anyway.  You only bought enough missiles to fill the silos of 3 1/2 Vanguards (Removing the missiles from the one of the four subs that was currently in overhaul.) and you weren't packing them full of warheads either, choosing to not bother with a full bus of 12 MIRV's per Trident D5, but only about 3.  If the RN is going to go from a 3+1 to 2+1 SSBN force, it can actually increase the number of warheads per active missile and still see a reduction in the total numbers.

[True Federalist (진정한 연방 주의자)]

I'd imagine that either England would concentrate all of its nuclear activities at Devonport or that they'd arrange to do more than just missile servicing at Kings Bay, Georgia, but also warhead storage and maintenance to replace RNAD Coulport.