One of the benefits of globalisation is that no serious, technologically advanced country can now stand alone. We're all part of a interrelated network of trade, supply, services and goods, all reliant on one other.
I'd argue that it's therefore inconceivable (within any short-to-medium term timescale) that we could see this country directly threatened (Paul's "invasion") by any other. For that reason, any "hot war"/"cold war" military force of a scale we've seen in the past is an unaffordable nonsense.
That said, there
are threats:
Peak OilPeak WaterHuman-influenced climate change
Ideological terrorism
Denial of Service cybercrime
Drugs and organised crime
Much of our current navy - or for that matter, the two new aircraft carriers - will not have an
essential role to play in most of the above. Worse, for those roles where we could see some benefit to having a fleet, there are other - often cheaper - methods of force projection and delivery.
We might like to think of those days where the British Empire covered the globe and was reliant on a large Navy, but it's a total fantasy in the modern age.
Andy