It's all about strategy. There are two main paths that any nuclear war could take: The first is all-out war, we're all screwed regardless of strategy. The se

It's all about strategy. There are two main paths that any nuclear war could take: The first is all-out war, we're all screwed regardless of strategy. The second is a more limited tactical war where the aim is to try and take out the oppositions nuclear capability first and then focus on their military. Unlike the USA, the UK has only a very small landmass to hide big ICBM sites and they will always be near some sort of major population centre. In a limited war scenario, it therefore makes sense to not have any fixed silos but to keep it all at sea and hidden. They may be able to nuke our submarine bases and catch the subs at rest, but we have a standing policy to always have at least one of the subs at sea at all times. I guess you could argue in favour of air launched missiles from the back of a carrier, but then you go to all the expense of maintaining multiple launch systems, etc. Even the current Trident system is frequently criticised for its cost, so that's another reason to keep it limited and simple.