It is not just Norwegian Air UK or Norwegian Air International that fit ALPA's definition of "flag of convenience". Swiss, Austrian, Brussels, Virgin Atlantic*, Thomas Cook, Aer Lingus and many others all fit that definition - being majority owned by an entity located in a country outside of their home country. There is no legal way to exclude just Norwegian, many other European Airlines will be negatively affected. And yes, that will start a trade war. Keep in mind too that Norwegian Air Shuttle, the Norwegian AOC, is in absolutely no way, shape or form, a so-called "flag of convenience".

*Virgin Atlantic is only 20% UK owned. 49% is owned by Delta and 31% is owned by Air France-KLM.