It is duty free to the seller who then increases the mark up so it is nothing like duty free price to you, the buyer.
A wenwe tale:
Wenwe were in the RAF, buying at overseas stations or from Saccone & Speed in Gibraltar, cigarettes were a sixth of UK retail (5p for 20 cf 30p in the shops).
I don't know what true DF spirits prices are now; perhaps someone who's bought from the aforementioned S&S may advise.
Just had a look at an old wines & spirits price list from RAF Gatow about 1970.
Prices are per BOTTLE:
Ordinary Whisky: £0.71
Glenfiddich: £1.29
Martell VSOP: £1.63
Vodka (40oz): £0.48
Chateauneuf du Pape: £0.50
