Agree that Article 46, at least to begin with, represents a "coalition of the willing" to coin a phrase.
Article 46 is quite clear all the way through:
1. Those Member States which wish to participate in the permanent structured cooperation ...
3. Any Member State which, at a later stage, wishes to participate in the permanent structured cooperation ...
5. Any participating Member State which wishes to withdraw from permanent structured cooperation shall notify its intention to the Council, which shall take note that the Member State in question has ceased to participate.
The EU army is Article 42 (2) and it is also unambiguous:
This will lead to a common defence, when the European Council, acting unanimously, so decides. It shall in that case recommend to the Member States the adoption of such a decision in accordance with their respective constitutional requirements.
And the UK’s “constitutional requirement” would be a referendum (as would Ireland’s).