Phil,
I used to export motorcycle parts to the USA using a range of commodity code, mostly 84799070 associated with "machine parts", without difficulty.
Allocation of HS codes is something of a game, as is the description used. The first line of the invoice is key, all invoices are machine read and it seems some words attract attention, whilst others do not.
I'm glad I'm retired...
It does seem that since BREXIT HMRC has become pedantic in the extreme, I've no idea why. A couple of my consulting clients are thinking of pulling out of distribution from the UK and moving to Ireland because of the delays and difficulties.
Write to your MP and evidence your frustration.
