it was against war with France that Britishness first defined itself - perhaps, or maybe England just happened to win that particular war. Was the war you had in mind before the uniting of Scotland and Engand? The Scots and French have formed alliances against England in the past, I think.
it was via the formation of the British empire that Britishness sought to identify itself - no, British identity already existed. The Empire sought to force-feed Britishness to aboriginal peoples.
The British Isles consist of Britain and Ireland, therefore Britishness/British culture is made up of Northern Irish, English, Scottish and Welsh cultures. Simples.
I don't think British culture is 'unfixed' but it sure is flexible.