The UK's exports are only around 58% to the EU and our imports only around 53%. We also have that unique and valuable institution called the Commonwealth. Clearly we have significant interest in World security beyond the EU borders. On that basis, it does not seem overly wise to align and integrate ourselves with EU defence policy and capability. What may be good for the EU's (I'm sure I voted for staying in the European Common Market) interests may not be in the UK's interests.
The entire concept seems to follow the economists' dream of a World without war and the saving of assets "wasted" in its conduct. Regrettably, it's not the blood of economists that is shed if the dream fails to become a reality.
Of course alliances are important and are a force multiplier. The contribution of the alliance members, though, must be balanced. It would be folly for a country in the UK's position to be dependent on another nation for a key force element. The point about support of an ageing population from a reducing birth rate is well made but I don't think the European model is the solution: exploiting and expanding our technicalogical advantage probably is.