SaN - no need to bow - I'm also a civvy and a former shipbuilder to boot. DIS only makes economic sense if you want to create a self-fulfilling prophecy. It is based on two questionable premises : 1 - that UK will require ever decreasing numbers of DD/FF, because 2 - shipbuilding costs will rise inexorably.
The first (from a commitments point of view) is far from certain, the second occurs precisely because UK industry does not get to exercise design capability on a sufficiently regular basis. This leads to cost explosion as shown by £600M per copy of T45 - which PAAMS aside is a pretty poorly equipped vessel. The procurement model for 45 is very different for that for T23, but it's so expensive because BAeS are having to acquire skills they have not had (feasibility design of small surface combatants). If DIS is implemeted, the next DD/FF design will be 2025 (just in time to forget everything learned for FSC).