Bigpants,
May I suggest that, in part, the answer to your question is the way the armed forces of the UK procure capital equipment and then support it in service.
If 'the system' took a 'through life' view of the procurement and ongoing support, plus the likely requirements for upgrades and set down a plausible replacement strategy - should that requirement have been identified, the outcome for many of our weapons systems would be very different.
The major impediment to realistic costings is the annualisation arrangements and the grubbing around which goes on just to keep a weapon system viable against all the other claims for funding. The method by which future costs are measured against inflation leaves something to be desired and this without political interference in the process (would we have bought SA80 if it wasn't seen as important to keep a factory open?).
Old Duffer