Airpolice,
Never mind all this crap about AFG being our highest priority - it's just a 'relatively' short term adventure. The primary task for UK Defence is the provision of an SSBN capability. All political parties have consistently signed up to this and as you are probably aware, whatever happens with SDSR, a Trident replacement is a given.
So if you start off with an almost blank sheet that has SSBN capability at the top, the next paragraph is that you need assets to support that capability - i.e, a secure base (Faslane), nuclear infrastructure, storage and support vessels etc.
Then you must have assets to 'defend' our ultimate deterrent, otherwise there is no point in having it, and this is regardless of what anyone considers the threat to be. Long range, fast and ASW capable = MPA. Full stop. In our case, having spent the money already, means we will have an MRA4 in service - eventually.
After all of the above, you can then step down to AD assets for the defence of the UK and roll on from there in deciding where the cuts should fall...