The Battle of The Atlantic, revisited.
And so we see emerging a 21st Century version of the Battle of the Atlantic, in which the sprightly EU-boats of Thomas Cook, Virgin, and now Norwegian are spreading panic amongst the lumbering N-registered convoys.
The Manchester / North of England market wants, above all, value for money. That is what the three above-named carriers offer - a good product for the market at an attractive price.
Delta will now use Virgin for MAN, UA Washington is gone (slots have been pulled for summer 17), AA are increasingly becoming a seasonal carrier using outdated equipment and with poor reliability (there's no 787 next year).
One question is how BA/IAG feel about reliance on a struggling AA to provide much of their market share ex-MAN.