Norwegian, like Ryanair and to a lesser extent easyJet are a trans European airline and it makes perfect sense for them and for many employee's to pay their taxes and social contributions in the country where they are based as required by European law.
I believe easyJet is still over twice the size that Norwegian is. With about 50/60% of its fleet based ex-UK.
Although easyJet pilots are heavily unionized, the matter is that IA in any continental base are broken by the use of pilots on a UK contract. They cannot refuse to operate under UK law.
The pilots in easyJet have already been split up in to many different contracts per country in order to comply with the law so that social security is paid in the country where the pilot is based.
I'd say that easyJet is THE example of a trans European airline. Unlike Ryanair who continues to employ people on contracts in Ireland.