It sounds crazy to you maybe, but there are mutual acceptance withing EU/EEA. Other countries have unilateral agreements of between Maritime Authorities. Besides, EU has a vetting system that act for all EU/EEA countries to check on institutions in third countries.
UK will probably soon be on their own, but traditionally they accept training and CoCs from other Commonwealth countries. (Maybe with a few exceptions) They MAY revert to that.
If you want proof I can link to EU, Norwegian and UK Maritime Authorities and Maritime Laws, but you may be an old man before you get through it all.
No you will not. This is not USA.
You cannot “just go and get a British or Norwegian license”. You have to have the required maritime education per STCW curriculum at an institution that is approved though the system described, in a country that is accepted by the Norwegian and/or British Maritime Authorities.
The normal time required is 2 years (2 x 10 months) in a Maritime School or Academy. or three years at a Nautical Faculty at a University to get a BSc in Nautical Studies.
You also need to pass mandatory examinations (which is NOT multiple choice) and serve as Cadet or Apprentice to get sea time before you can be issued by a CoC as a Junior Officer/Engineer.
“Hawespiping” in the US sense, where you can self-study and take short courses and test at different times is not offered in Norway or other European countries. (No sure if the UK still have remnants of their old Cadet system)
If you hold a CoC from one EU/EEA country you can serve of ships of any other member country, although there may be some restrictions for Masters, but dispensation is frequently given.
Vessels under one EU/EEU flag can also trade freely in other member countries, much like ECO can operate in Alaska.
Sounds like utopia I know, but the rest of the world is not necessarily like USA.
BTW; most European countries manage to maintain at least some of their fleet under their national flag, with some national seafarers still employed to maintain the maritime knowledge required.