Just been trying to work this out. I think I've got it right...
If you import something you have to pay import VAT and customs duty if it's over £135. But you can bring in £390 worth of stuff if it's for personal use. So if I buy something for £390 online from the EU and want it posted, it'll cost me nearly £500 with import VAT, customs duty and handling fee, before the seller knocks of their country's VAT. In which case, it's going to be cheaper to fly over there, buy it myself, and bring it back in hand luggage! Once we're allowed to fly, of course.