IMO, the only justifiable, defensible, military defence force is one which is large enough to keep out those who would seek to invade, and its only justifiable, defensible, location is just within the borders of the country it defends.
Thus the British Army should not need to be outside the coast of the UK to defend it.
Any further out is aggression, not defence.
Not even the politicians who try to sell the concept to a gullible public can really believe that our boys are defending us in Afghanistan.