My understanding
On jan 1 2021 UK Licenses become ICAO . There will be a 2 year transition period for EASA Licensed pilots to fly G reg aircraft . That ends after a 2 year period . It is by no means clear what the future arrangement will be . Norwegian Airlines have a number of pilots with UK Licenses on their payroll . Many of their aircraft have EU registrations . The CAA have just appointed a CEO with no airline experience on 700 K a year . I would expect an announcement fairly soon to clarify the situation .