I always ask first and most places are fine with it . If you don't ask you don't get ! We were flying out of Heathrow in May and sitting in the departure lounge I was desperate for a draw , getting more and more ratty as we had over an hour before boarding and an 11 hour flight ! "Sod this !" Me thinks , so I went and asked a guy in one of the many duty free shops if I could go and vape anywhere and to my complete surprise he said "as long as your sitting down " you can vape ! I nearly fell over ! I'd already been sitting around for about 2 hours desperate to get my nicotine hit and thought you couldn't ! BUT YOU CAN !
That was terminal 3 departure lounge @ Heathrow May 2016 . I asked 3 staff just to be sure and they ALL said the same .... Don't walk around doing it but it's fine to sit and vape!!!!
Must admit though , I felt really bloody self conscious as I was the ONLY vapist in the place and I got starred at and glared at by many passengers but a vaper has to do what a vaper has to do and do it I did !!!!!!! Ahhhhh nik o'teen, my friend !
So moral of the story is ALWAYS ASK ! You may be very surprised!