Britain is pretty lucky, it has a strong central government with a loyal military and competent regional administrative bodies and police forces. It can survive a degree of decentralisation because of this. moreover, the prerogative can be used quite extensively. it seems the system can change without a serious constitutional crisis - at least thats what i get from the series. the government alters itself without seriously altering its character.
I think that it very much depends on the pre- and post-war situation of the country in question whether a federal structure is a good thing. I think that it needs a balance to function: the less ethnically and socially centrifugal forces you have within a state, the further you can go with de-centralisation without endangering its existence. As long as you are not crossing that line of balance, de-centralisation is most probably an asset in such a time of crisis.
Thinking of the US of A. You are right with your observation. In the long run, though, I would think that the identification with the USA as such is stronger than with the single States. While a renewed USA might be more federal (i.e. also more similar to their original design), most surviving states will re-unite or even pretend they never split apart.
New constitutional orders will appear everywhere, this is almost a natural thing when a nation faces crises of such a magnitude. (Germany 1918/19,1948/49; France 1870/71, 1940, 1944/45; Russia 1917 and so on)
This got me thinking about how Australia would organise itself in such an emergency.
I think that Australia will most probably be "lightly" hit. Large tracts of land with minor fallout issues. Large agricultural basis. In this scenario, there is plenty of warning time for anything valuable to disperse.
I assume that the Australian government even remains intact under such conditions and is in command of the situation after a few days.
hopefully either melbourne or sydney doesnt get nuked. we need at least one of those cities.
I fear that both cities might be targets even if only a dozen hits get on the continent. However, I recently read how huge Sydney is territory-wise. Just putting one warhead on it might not even eradicate the place.
Just a side note, i hope Bob Hawke survives D-Day but Keating and Howard get vapourised
Ronan Keating? Just kidding.
Though I am in no position to judge Australian politics, that's a bit mean.