Can you ring up the duty-free shops at Heathrow to see if they stock a certain item you might want to buy ? Or is there any website which lists goods available there ? I don't mean the known chainstores like Boots; just the big areas which sell perfume, ciggies, booze and fiendishly expensive handbags.
Or google "world duty free" for info at the different terminals.
BUT remember that the "savings" are against normal full High street prices...and who's daft enough to pay them?
What's in stock now might not be in stock when you fly out or back.... and stuff's often cheaper in the shops abroad, and things like perfume certainly in the shops here or on-line.
I haven't bought anything in duty-free for years (not that we ever fly anywhere these days that's not EU, apart from the Canaries, so duty-free doesn't really apply to us). Where are you going?