The debate around a £30 billion replacement for the Trident submarine system is drawing in some powerful allies on the side of those looking to scrap Britain’s independent nuclear deterrent.

The latest in a growing line of senior military commanders (retired) calling Trident an expensive irrelevance in a modern world is Field Marshall Lord Bramall.

From a purely military point of view he and his supporters are probably right. Such a system of mass destruction is to all intensive purposes unusable.It is inconceivable that the American’s would allow Britain to use a weapon without their say so. So the independent part of the deterrent is actually a misnomer as well

And that is before even considering who such a weapon would be used against. It would take a scenario planner of extraordinary imagination to come up with a feasible situation where such a weapon could conceivably be used.

But the military strategists who are turning against Trident are missing the main point. Trident isn’t a military deterrent; it is the UK’s most expensive PR tool.

Thirty billion doesn’t just buy a weapons system, much more importantly it also buys a continuing presence as one of the world’s decision makers.

It won’t be the MOD who finally signs up for the replacement, for there is a growing lobby not just among former senior officer who would far rather spend the money on other kit, it will be the Foreign Office.

For Trident has little to do with deterrence or the projection of effective military power and everything to do with Britain’s position in the world as a ‘player’.

With the growing influence of countries such as Brazil and India not to mention the 60 year absence of Germany and Japan there is an growing body of opinion for an expansion of the UN’s five member security council. If Britain was no longer a nuclear power what would be the argument for continuing to keep her seat at the top table?

Many of the growing band of developing countries knocking on the security council's door would be only too glad to see a small group of islands in the north Atlantic relegated.

Having a UN Security Council veto is a phenomenal position of power to have and it is inconceivable that any UK government of whatever colour would do anything to put this in jeopardy.

If those arguing for Trident’s scrapping are happy enough to see the UK downgraded to a second tier European power along with Italy and Spain then fine.

But they need to be up front in accepting this arguement and admit that a Trident replacement is a diplomatic purchase, not a military one.