Somewhere under the Atlantic, one of the U.K.'s Vanguard-class ballistic missile submarines is on its secret deterrence patrol, safeguarding British sovereignty with its Trident missiles. But on the surface, the future of this deterrent is facing very public scrutiny as the government prepares to lay the foundations for its replacement.

The government wants to replace its four ballistic missile submarines (SSBN) with a new generation, and new warheads for the refurbished Trident missiles, but the cost of the program—estimated at £15-20 billion ($23-30 billion)—is a difficult one to swallow as the country tries to reduce government spending, particularly for the armed services.

The current coalition government, formed by the Conservatives and the Liberal Democrats, have pushed the new SSBN program—called Successor—ahead, with the signature of contracts worth hundreds of millions of pounds. Even before a final “main gate” decision on the development of Successor, due in 2016, the government is due to have spent at least £3 billion.

Now, a review of alternatives to Trident has been published—in a redacted form—prompting discussion of the program's future. Publishing of the review was called for by Liberal Democrats, who have long believed the price tag for the deterrent is too high and that the funds could be best spent by better equipping conventional forces.

With four submarines, the U.K. is able to maintain a 24-hr. deterrent, with submarines in dock ready for deployment and another in refit or maintenance. But the Liberals believe the mission can be achieved without “continuous at-sea deterrence.”

In a speech to the Royal United Services Institute on July 16, Danny Alexander, Liberal Democrat chief secretary to the treasury and minister in charge of the Trident alternatives review, said that the review process provided a “real opportunity” for making long-term savings and “recalibrating our nuclear weapons policy to the requirements of our age, and to contributing to nuclear disarmament.”

While the Liberals do not have a policy of unilateral disarmament, their stance suggests that while the U.K. is not at risk of surprise attack, such as during the Cold War, the new Successor submarines could be held at readiness rather than sitting on a “hair-trigger.”

Procuring one less Successor submarine would produce savings of about £4 billion over the life of the system, Alexander says.

The U.K. has reduced its arsenal of nuclear weapons considerably since the height of the Cold War, removing its last air-dropped WE177 in 1998. The country now has 225 nuclear warheads, one of the smallest nuclear arsenals of any of the declared nuclear powers, Alexander pointed out.

“We have the ability to step down the nuclear ladder when we find the political will to do so.

“And the next great big step down the ladder is to reduce the salience of nuclear weapons in our Defense policy itself. And that means accepting that a Cold War-style continuous deterrent has become unnecessary,” added Alexander.

Ministers and former military commanders said that ending the deterrence patrols would be “weakening our national security” to save a “very small fraction” of the defense budget.

The review supports the purchase of the full fleet of Successor submarines and the retention of the continuous deterrence patrols, pointing out that the development of a new independent warhead for a submarine-launched cruise missile, fired from a modified Astute-class SSN would probable take more than two decades, long after the planned retirement of the Vanguard vessels.

Consideration was also given to an airborne deterrent capability dropped or launched from a large aircraft or the F-35 Joint Strike Fighter, allowing strike from the U.K.'s new aircraft carriers and land bases, but the report considers air-launched options too vulnerable to pre-emptive attack even if the aircraft are fitted with stealthy or non-stealthy nuclear- tipped cruise missiles.

None of the alternatives could “guarantee a prompt response in all circumstances,” the review stated. It also said that transitioning to any of the “realistic alternative systems” was now more expensive than a three- or four-boat Successor fleet.

The debate comes as the U.K. also considers what it may have to do if Scotland declares independence from the U.K. Naval Base Faslane on Gare Loch has been home to the U.K. ballistic missile submarine fleet since the age of the Polaris, but Scottish nationalists have a staunch non-nuclear policy and have stated that the deterrent ought to be removed. But with no place in England or Wales to harbor the submarines, the U.K. would be forced to unilaterally disarm if this scenario comes to pass.

Some reports have suggested that the government would try to make the base sovereign U.K. territory, similar to the Sovereign Base Areas established in Cyprus. However, the move was denied by the government, as it faced a backlash from Scottish ministers.

The Scottish nationalist government hopes to hold a referendum on independence in fall 2014. If the people vote “yes,” it is the administration's intention is to have a constitutional platform in place for Scotland to become independent by March 2016.