Just checked with HMRC.
Goods bought in duty free when traveling to a non-EU country are subject to import duties (if applicable) and VAT when you return if they cost more than £390= on the full amount paid.
Further, and I did not know this, if you spend more than £390 you have to pay VAT etc on the full amount spent. They do not take off the £390 and charge you on the difference.
I think the message is to stick to the High Street!!!