Our nuclear submarines, the great deterrent fleet - assuming it is as undetectable as claimed (theres probably many ways to detect subs from space for example, checking for magnetic anomalies which an advanced enemy would be able to do) the expensive subs and their deadly weapons that will never be, or only pointlessly used, circle our oceans on patrol against enemies so far not imagined.
Getting rid of trident doesn't deal with the possible truth that one day we could have an enemy that would wipe us out, but for our strategic deterrent. I recognise this as possible so suggest an alternative to trident that is more productive, positive and good for us and mankind: A space program aimed as UK resilience to total anhileration. Such a program would be expensive, but so is trident replacement and would achieve the same goals in a different and peaceful way - instead of revenge as our deterrent - resilience becomes our deterrent - since resilience through solar system colonisations (and exploitation potentially of its resources to great profit) would thrust britain into a position where she is difficult to destroy by a crazed enemy. Such an enemy would have to factor in the possibility of future revenge from remnant Britain from the solar system colonies.