If you have nucelar weapons then you must be prepared to use them. Whilst some would argue the lunacy of Mutally Assured Destruction kept the West safe from the Red hordes during the cold war, the principal simply does not make any sense in the current global political and military climate.

In a scenario where a successful pre-emptive attack was launched on the Britain by (for example) Iran using nuclear weapons and they managed to explode a warhead on a British town or city, would the response of then launching a nuclear strike on Tehran be justified? Given the loss of life would be almost completely civilian, can the use of a nuclear retaliation over conventional be considered? Is killing more civilians the correct course of action to take when the civilian loss of life would already be catastrophic?

If there is a nuclear threat, it'll come from a terroist with a suticase bomb rather than the unlikey launch of an attack from Iran or North Korea, and no amount of expensive hardware silently stalking out in the Atlantic will prevent that.

Remember, Geoff Hoon (a man so devoid of moral integrity he makes Norman Tebbit look like a hand-wringing liberal) said the government would not rule out using nuclear weapons on the battlefield - if we replace Trident, then we give the lunatic the button to press.