DB
You know the answer to your own question, but let me answer it for you.
Pilots will be employed by OSM Aviation on permanent contracts, OSM Aviation is 50% owned by Norwegian.
Pilots will be subject to the labour laws of their country of employment, so if you are based LGW or EDI it will be (is) UK labour law.
This is not a FR type self employed/Director of some obscure Irish off the shelf company, you will pay all relevant taxes per your country tax code and national insurance.
The road show in Edinburgh is part of an ongoing recruitment drive for both TR and NTR Captains & First officers.
It is intended that EDI will be a 738 MAX base only at least to start with, the road show is not about EDI base.