Because they have a skirmisher defence under VCRA 2006 and that's all that is required to be in compliance with customs legislation with regards to importation laws at the border. A lot of people seem to have the impression that being UKARA registered is the only defence and it isnt at all, its the easiest one to establish immediately but that's why goods can be detained under Customer Excise Management Act 1979 for 28 days while an importer provides evidence of the legality of their import.
Someone suggested using an ATA carnet which they could get form the Norwegian chamber of commerce (or equivalent) and that could be used but it would only show the goods as being a temporary importation an is mostly used for goods over around £1500 if memory serves me correctly and is usually used for people travelling with valuable goods that they use for or with work or they would be stung with import tax on every arrival in the UK carrying the goods dependant on the circumstances
The OP is right though 'transitting' relates to people not goods