Just a point on the numbers; the £15-20bn is the 2006 White Paper costings for the capital costs of replacing a four boat fleet. It doesn't cover the running costs of the existing boats and the operating costs out to 2040ish. See p.7 of the White Paper (it's a .pdf on the MoD website).
In other words, the £15-20bn figure is disingenuous.
However, it is also disingenuous to claim that scrapping Trident will save £100bn soon. It will, but not quickly - the capital spend will run from 2012 and 2027 (White Paper, p.7), and the ongoing running costs will be spread as well.
So the economics of the question is not a show stopper - it could be afforded. But £2-3bn a year in replacement costs - plus the costs of Aldermaston (£750m p.a. in 2007, according to
House of Commons - Defence - Written Evidence at note 83), Faslane and Coulport (no idea), then the cost to defence is likely to run to £3 - 4bn a year in the middle years of the next decade.
Given the pain that we're going through to make savings to balance the current budget, if we don't need the deterrent, we should take the savings, and take them now.
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