The primary purposes of a State are to defend its borders against outside intervention and to maintain the peace within those borders. All else - Health & Education included - is secondary. A Minister who says that money spent on defence is wasted is, by definition, unfit to hold any position within government.
We do not need to re-think our role. This is why:
We DO need to rethink our role. The British Overseas Territories that Widger mentions are part of our State and fall within the primary purpose of defending our borders. Iraq, Afghanistan etc. do not.
...maybe we will fundamentally disagree about the value of trying to stop a rather nasty internal conflict. In those situations we will need proper UK armed forces.
If China invaded Taiwan or Russia invaded a Baltic State that would have nothing whatsoever to do with us. Our purpose as a State is to ensure that China or Russia or anyone else do not violate our own (including those overseas dependencies) borders.
Which brings us nicely on to the next point:
So, is Trident - and its upgrade - really needed?
The purpose of Trident is not as so many seem to think, to guarantee our seat at "the Top Table". It is there as a deterrent. It replaced the Polaris that replaced the "V" Force that replaced the truly massive armed forces that existed at the end of WW2. The deterrent that some of our ministers seem to think is a waste of money. Those who say our deterrent has "never been used" don't understand the purpose of a deterrent - certainly Nikolai Valuyev has never been mugged.
It is the Force Projection plans that need to be done away with. Our defense should be limited to just that. Defending ourselves against external aggression. Being aggressive towards others or joining in their squabbles is not our business. In fact, what I suggest is isolationism. There's nothing wrong with minding your own business when there's bigger boys in the schoolyard.