It doesn't have to be this way.
Since the item was being replaced for free - I don't understand is why Seagate's UK subsidiary couldn't pay the VAT and reclaim it.
I recently bought a music thing for a German retailer, who have a UK store. I paid UK VAT at the time of purchase
The item was shipped directly from Germany. Delivery was about a week later than estimated due to "brexit related issues", which is better than I expected. I didn't, however, have to pay VAT again, and I didn't have to pay an admin fee to the courier.
Clearly the vendor has got all this sorted out. Apparently they previously had some issues with the forms they had to fill out, but they were resolved by the start of February.
If this German company can fight it's way through the quagmire of Brexit, so can anyone.