I think it's Thomas Cook that have kept Norwegian away from MAN.
Based on the current offering at LGW, likely Norwegian routes from MAN would have been, at best, Boston, New York, Fort Lauderdale, Los Angeles, Orlando and possibly Oakland. It's worth noting on their website, they sell Fort Lauderdale as Miami also.
Thomas Cook already operate all those routes, barring Oakland, but word is, San Fransisco will feature in the Summer 2017 programme.
Thomas Cook also have a bigger Caribbean programme from MAN, and are also rumoured to be starting Tampa.
So, what could Norwegian do? TCX's fares are well on par with Norwegian, but TCX have volume on their side, using the A330 versus the B787. There's also worth considering BOS and JFK from Norwegian could have been B737-max routes, so, TCX win hands down on volume there.
TCX are also rumoured to be looking at wifi for the A330 fleet, and offer basic connections via other carriers. Therefore, it seems any advantages Norwegian had, the gap is being closed.
Europe wise, Norwegian would be late to the game from MAN. Not saying they couldn't carve a market, I'm sure they could, but, they have a lot easier task at BHX than MAN.