"it's well established that there are plans to use the army post-Brexit in the UK if necessary."

Governments plan for all sorts of contingencies and actually wargame some. The US had plans to invade Canada
to plan for something is not necessarily to mean it will happen.

Corbyn wargamed for a catastrophic drop in the value of the pound, should he become premier. The argument over wheter that was merely precautionary or mandatory I leave to others!