Yeah it depends on which terminal you end up at Heathrow, but for me last year my return flight to the US was delayed by 3 hours and was lucky to be at a terminal (don’t remember exactly which one it was) but it was pretty much a shopping mall and some good watch eye candy. I remember seeing a white dial Rolex explorer II at the window and it was gone within the time I was there. I however, didn’t think the prices were any better or worse than those in the US considering exchange rates at the time.