I hear your pain although the majority of the issue is the shipping company. As far as I can see, the only issue with Kemper is they classified it as a sale and potentially sat on a correspondence ( but the shipping company were also talking to you).
Once shown its a repair, should be easy to re-classify it.
You should expect that they know what they are doing. You are not an expert in shipping and shouldn't have to be.
Kemper - although this isn't your fault, it certainly isn't your customers fault and will affect their interaction with you. Brexit has made any transactions harder/more complex. You do ship to the UK and potentially have more knowledge although agree its not your responsibility.
I have resisted buying direct from Kemper now because of the confusion but I hope it settles down.
I would suggest perhaps:
1) Recommended carriers
2) Advice/approved repairers in the UK
3) Some basic guidance and support