At the moment, there's no guarantee that Norwegian Air International will be able to fly transatlantic from Belfast or Edinburgh after March 29th. That could be a factor.
Its possible, but Gatwick flights remain on-sale.
Norwegain, itself, is registered in Norway so dot directly affected by Brexit. Norwegain has a series of Airlines, registered in the U.K., Ireland, Argentina and elsewhere. Brexit might play in, but not majorly.