Aer Lingus or Norwegian the customer base is pretty much the same.
Except Aer Lingus passengers weren't originating from Scandinavia, so it's slightly different and also it looks as though they'll be feeding from those Nordic destinations.
It will take some HEAVY marketing to get the UK general public to think Norwegian = Arrecife or Pula once or twice a week.
That's if you're looking to attract just UK customers. As you said, those routes are heavily served from LGW, but the load factors are pretty high. IF Norwegian are looking to feed from Scandinavia, then any remaining seats could be mopped up with UK passengers, without HEAVY marketing.