"The three-stage, solid-propellant, inertial-guided ballistic missile can travel a nominal range of 4,000 nautical miles and carries multiple independently targeted reentry vehicles."

The whole point of a submarine-launched ICBM deterrent is that it is the most secure leg of the nuclear triad, the part least likely to be taken out by a massive enemy first strike and best able to respond with devastating force against an aggressor, even if it suffers high attrition during the initial hiostilities.

Not only that, it is also capable of a measured reaction, you need not use them all at once, but can ratchet up a nuclear response in the event of a protracted conflict, pressuring your opponent into modifying his attack. They can also be held in reserve, for months if necessary, so that the submarine commanders can make absolutely certain they are responding to a full-scale attack on the American homeland, and not an accident or relatively minor incident. Even the threat of doing this is going to give a potential aggressor second thoughts about starting something.

This type of weapon system need not be highly accurate, since you'll be hitting cities, not hardened silos (which will probably be empty anyway in the event the enemy has initiated nuclear war). Consequently, you can afford to make the missiles robust and cheap, and build lots of them.

But highly accurate warheads, particularly multiple independently targeted ones, are purely a first strike weapon, especially when launched from just offshore so the target nation has little time to analyze the attack and decide how to respond. I can see no other reason to go to the expense and effort of giving them these capabilities. Putting them on a submarine is extremely expensive when big dumb city busters would have fulfilled the deterrent mission much better and more reliably at a lower cost. They are also ineffective for first strikes, making them less threatening to an enemy not already contemplating to shoot first.

Highly accurate mirved weapons are designed to confuse and penetrate missile counter measures and take out second strike weapons the enemy is relying on for their deterrence. Launched from offshore, they maximize the number of strikes while minimizing the nuymber of detectable launches. They send entirely the wrong message, a message I picked up on when I first heard of their capabilities soon after their deployment. This is exactly the kind of weapon a nation who plans to start a war would design. What could we possibly have been thinking?

This system may have been a technical marvel, but it was highly destabilizing and extremely ill-considered. It is almost deliberately designed to make your opponent trigger-happy, and inclined to shoot first and ask questions later in the event of an accident or confusion during times of high tension.. We're lucky it didn't get us into a war with the Soviets. I felt that way when it was deployed, and I see no reason to feel any differently now.

The fact this system still maintains its capabilities and high reliability means none of these shortcomings have been lessened. Giving them to an ally capable of independent use, even a trusted and responsible one like the UK, only multiplies these shortcomings further. And of course, against the kind of enemy we are more likely to encounter in today's world of terrorism and small rogue states, it is total overkill and useless as a deterrent.