Duty free pipe tobacco at LHR is only available at Terminal 5. I've written about this extensively in previous posts:
http://pipesmagazine.com/forums/topic/condor-at-lhr
And if you are vacationing in Thailand, here you go:
http://pipesmagazine.com/forums/topic/pipe-tobacco-retailers-in-bangkok
And should you be going to Barcelona anytime soon (didn't post about duty-free, but I remember seeing mainly MacBaren tins of Plumcake and Scottish Blend going for cheap; while this not be a big deal to US travelers, Canadian, Aussie, NZ pipers, among others, might take note ):
http://pipesmagazine.com/forums/topic/pipe-tobacco-retailers-in-barcelona-spain
Just got back from a trip to Rome and need to post about tobacco shops there with pictures. However, I did not see any pipe tobacco at FCO--it was mainly cigarettes and Toscano cigars (mind you, I was flying through a mainly European terminal and duty-free is not available for travelers going to and from EU member states). As Switzerland is not a member of the EU I was able to pick up some duty-free goods (mainly alcohol, which I will also discuss in more detail in the upcoming post).
Dubai duty free has TONS of tins of Dunhill--mainly Nightcap, London Mixture, and EMP; and Davidoff Green, Red, and Blue Mixtures. Also sell Peterson Pipes but can't remember the prices.
Abu Dhabi duty free sells Dunhill tins and pipes; Picked up a tin of Royal Yacht there a few years ago but haven't popped it yet--its back in storage in the States.