Your goods will be qualifying as Northern Ireland goods from the first of January 2021 if they are in free circulation in Northern Ireland – that means not under a customs procedure or in an authorized temporary storage facility – before you move them from Northern Ireland to Great Britain (England, Scotland, and Wales).
Goods starting their journey in the EU will not qualify for unfettered access if they are moved through Northern Ireland into Great Britain for an avoidance purpose.
If you move qualifying Northern Ireland goods directly from Northern Ireland to the rest of the UK there’ll be no changes and no new customs processes for almost all traders, with some very limited exceptions.
If you move qualifying Northern Ireland goods from Northern Ireland to Great Britain through Ireland you’ll need to follow the process for importing goods into the UK from the EU unless you are using transit but you will not need to pay UK customs duties.
There are some limited exceptions when you need to submit an export declaration for a movement of goods from Northern Ireland to Great Britain. These are if your goods are:
Placing qualifying Northern Ireland goods on the market in the rest of the UK
You will not need any new regulatory approvals to place your qualifying goods on the market in Great Britain.
Source Credit: gov.uk