As things stand, that's not a bad set of priorities
Perhaps, I think maybe it depends on ones foreign and domestic security policy objectives. Here is a (viable) alternative:
1. The defence of the UK.
2. The permanent 4 boat deterrent.
3. The optimisation of homeland security.
Not saying I agree with either one...simply stating that there are some pretty successful nations on this planet that seem to survive and flourish with much simpler (less expensive) military hardware.
Of course most of them don't possess an IND, but p'haps we call all agree that is a slightly different dynamic.