Have just returned and would say that the cigarettes appeared to be cheaper upstairs at the gates as opposed to downstairs in the duty free.
Also, you only pay your US$20pp departure tax at the depature gates, you don't need your passports just one person from your party to take all your boarding cards and the money, they then stamp both sections of your boarding card. This wasn't explained to us, so was a little confusing at the time as the arrival tax is dealt with diferently to the departure tax.
Lastly, at LGW this morning our flight was the highlight of the morning for the customs guys, and they had a huge haul in cigarettes. I understand you can only bring in one carton of cigarettes per person and lots of people had obviously bought cigarettes whilst in resort and put them in their checked in luggage, this luggage has obviously been scanned and customs were waiting for those cases and their owners!