I just started doing OA in the UK and sent in a shipment a couple of days ago. The same day I sent it, I received a Compliance Requirement for GFSI or HACCP documentation for some supplements. It’s a well-known brand, purchased directly from them, and there are six other sellers offering the same product. I’ve reached out to the company, but they won’t share those certificates with anyone. I tried to appeal, but it got rejected.
They are not obligated to give the information to you, especially if you don’t have an agreement with them to sell on Amazon. So if they won’t, you have to move on and try something else to sell.
Unfortunately, dietary supplements are a very tricky category and as @AmazonUKForumRefugee says, the manufacturer is not required to provide you these documents.
If you really want to sell supplements, it would make better sense for you to launch your own brand.
Try to see if they are willing to let you return the items back on the grounds that you are unable to sell them without these certificates.