Hi Guys.
This has been going on for years before brexit. Postal and Courier always pay duty upfront whether you ask them to or not then hold your item to ransom with a "Admin" charge claiming it's for your 'convenience'.
Warranty repair items to the UK are or should be covered under the simplified declaration process.
European companies have not got their act together yet and have to register their warranty process items and sign a declaration prior to them being sent so custom officials let them through. This should only apply to warranty items, which should also cover replacements, which DJI likes to do.
I used to import stuff from the States and used to get charged an 'Admin' fee from any courier company for handling my parcel and paying customs fees on my behalf, sometimes when there was no duty to pay.
What the company needed to do, at my request, was to write on the customs form "goods to be declared by importer"
Typically in the states, no one has heard of this and sometimes would refuse to do so thinking it was some kind of illegal scam on my behalf. HMRC and Borderforce actually had the information listed on their website for some time clearly stating that courier companies were not allowed to make a charge if this information was written on the form.
Typically there is no space to write this info and many courier companies would claim that it is written on the box or over the form or near the form was not good enough. And tried to charge.
I spent years talking to my MP and even BorderForce and lobbying parliament (of which I am still due a reply) regarding this situation.
Border force and HMRC for a long time changed their web pages to omit this info and carried on their merry way.
It is back now and can be found here.
Tax and customs for goods sent from abroad: Documents - GOV.UK (www.gov.uk)
I have no longer need this as I no longer import stuff, but DJI should sort this simplified declaration prior to posting, and you, as the exporter/importer, has a duty to declare the warranty repair to the HMRC and repair company to make sure the repair company follows their due diligence so you do not have to pay customs fees.
If you are out of warranty then you most likely will have to pay. Sorry. Find a local company to repair your item.
There are many popping up and providing a good repair service in the UK.
Best of luck. I hope some of this info helps.