i thought the squad analogy was spot on. you have a number of submarines that carry a number of missiles, each with multiple warheads (a dozen per missle?). so of that 200 only a quarter tops are on the pitch so to speak, while the rest are on rotation (incidently the army is big on rotation...
no it wouldn't, the costs of Trident you note are a projection over its entire 30 year life time. it might therefore be said to buy a bit of welfare in the future. for reference, over the same period welfare will cost £3300bn, assuming no increase.