Custom duties are still collected at the time of delivery in UK. Not sure about VAT being collected by seller....I would think it is wise to charge duties on landing instead.
Do you have an account of this posted anywhere to read? I had to return something abroad for a warranty claim and I got charged customs for the replacement as they considered it to be new. Luckily the defective item came back to me as it was refused by the destination country since it didn't have bill (how am I supposed to issue a bill for that), so I can at least use its parts for spares.Previously, I had custom duties charged for a $20 item that was overvalued at Mumbai customs. I raised a complaint in the portal, the assessment guy called me, apologised and refunded the custom duty out of his own pocket as it was an under warranty replacement. So the complaint system works at least if rules are not followed.
Else I would request you to post a new topic for everyone's benefit on making international warranty claims without paying customs twice.