where are you flying from?
generally duty free not available from the EU, but in certain countries (Spain, Netherlands, etc) duty paid prices represent a saving over uk high street prices. From outside the EU duty free available.
HOWEVER in both cases you need to check whether you can carry purchases on to the aircraft at your point of departure. (For instance, if you bought 1L of whisky in a USA airport, you'd have to drink it before getting on.) In Europe the security measures at most airports are as per UK, but there are exceptions. Check with your airline.