I’m afraid I don’t have the answer to that, I have been too long away from Norway and Norwegian shipping to have followed things that closely.
But as long as Britain is member of EU I would think a valid COC issued by British Maritime Authorities could be convertible, or that a CEC based on such COC could be issued to a Norwegian citizen to serve on Norwegian flag (NOR) ships.
For NIS vessels, or Norwegian owned vessels registered in IOM, any British dependency, or even other Commonwealth or EU countries, (like Malta etc.) there should be no issue. As long as you are Norwegian citizen and/or resident there are agreed terms applicable for both member and non-members of the Norwegian Officer’s Union. Such terms are acceptable to other Norwegians so they can’t be too bad.
I would recommend you contact NMA to enquire, though. But act quickly, since UK may be crashing out of EU without a deal next March.
Here is contact details for NMA: https://www.sdir.no/en/about-us/contact/
Good luck Robin and don’t worry about that grumpy old CCaptain, he just enjoy being the big bad wolf on this forum.