I've wondered about that. (I think I asked the question somewhere on TP a few weeks ago.) I'm very tempted to give it a try.
If it works, I might save a lot of money. I'm VAT registered, so if I make a voluntary declaration I can just claim back the VAT anyway and I can't really lose out. (Except for a little bit because HK prices are a bit higher than UK prices, ex-VAT.) But as I understand things, DR can't actually refund my VAT because I won't have paid them any VAT. All they can technically refund is an amount equal to the VAT I pay. (A bit like some car adverts you see when their special offer is that they'll pay your VAT; the small print always says that it’s actually a discount equal to the VAT amount, but VAT is still payable on the reduced price.) So if the plan works, I'll get a big refund from DR and I still won't pay any VAT and it will all be 100% legal.
What do you reckon? Worth a try?