The deterrant force as it stands represents a cost that we cannot afford.
The Lib Dems are not advocating unilateral nuclear disarmament, merely that we cannot justify the cost of replacing the current system. If at some stage the economy picks up and as a Nation we can justify spending money on a class of submarines with no other task than lurking in the ocean depths, then so be it.
The use of attack submarines with cruise misiles offers a level of deterrance more appropriate to current world issues.
The argument about losing our military industrial capacity hardly holds true. Nobody is suggesting not having nuclear powered subs in the Navy - sub construction would not cease. We haven't built an aircraft carrier since WW2, but that isn't stopping us designing and building a new class of carriers for the 21st century..

