
There is no military threat to Britain that is not also a threat to Europe as a whole. We have alliances because of this fact. The Falklands was the last time we needed to defend our interests (as we saw them) unilaterally and nothing has materialised in the decades since that we will need to arm ourselves against on our own.
Let’s stop pretending that for Britain to determine a Defence budget is a decision we can make in separation from (at present) our NATO partners. Whilst we have NATO there is an obvious need is to revise its funding mechanism more fairly. The United States does bear a disproportionate burden and other members, including Britain, do need to pay their way more fairly.
It may be that an increasingly isolationist US will lead to the disappearance of NATO. This is not necessarily a bad thing. A united Europe does need to defend itself and the emergence of a European Defence Force (EDF) seems likely and logical.
The funding of an EDF is obviously open for discussion but a GDP related mechanism is a start point for these discussions. Similarly there will need to be agreement on bilateral deals between the EDF and friendly allies hopefully, but not necessarily, including the US. Other non European NATO partners like Canada will certainly be candidates for mutually beneficial alliances.