I agree that Heathrow is the best. I just recently purchased the Dionysus Leather Mini Chain Bag which sells for $1450 in US. I paid about $990 on my way out of Heathrow. The salesperson in the store told me they are the cheapest in the world. That was AFTER I bought the bag.
Also, I emailed them a few days before our departure to ask them to set the bag aside, which they did. I had stopped in the store on the way into the UK, and the girl gave me her card.