First off, let me direct you here
I cannot definitively tell you what you need to do to have your withholding tax reduced so I will be happy to opt out if at the end of this you consider that you are no further forward. If I opt out, another expert can take a look at your question though I'd be surprised if anybody else will be able to take you any further forward. We predominantly provided UK tax advice on this site.
It would appear that you need to register your company with the Norwegian government since you are working in their waters, albeit not permanently, and the links to the relevant forms are in the document I directed you to. How that will impact on your tax situation I cannot say. Your company will pay corporation tax in the UK and credit will be given against your UK CT liability for any tax paid in Norway by the company.
There are several recommendations I would make:
1 Find out from your colleagues on your vessel what they did to minimise their Norwegian tax deductions and do the same.
2 Appoint an accountancy firm with international tax experience.
3 Talk to Goss about your options and ask them what compliance procedures you need to go through to minimise the withholding tax deductions. They must have placed others in Norwegian ships and, given their apparent good reputation, they really should have given you chapter and verse when they took you on as a client.
4 You should call the UK tax office and ask them to write a letter confirming that you and your company are UK resident for income tax and corporation tax purposes respectively.
5 You might get some advice from here
but I'd get what information and advice you can from your colleagues and Goss before you go elsewhere. Other British workers on your ship should be able to recommend tax and accountancy services.
Let me know if you have any further queries.