If you travel to another EU country you can purchaes cigarettes for your own use without restriction and without paying UK taxes when you return....but they HAVE to be for your own use (or immediate family). HMC&E have decided that a "reasonable quantity for your own use" would be approx 3000 cigarettes.
It has recently been argued in EU courts that this should also be applied to cigarettes purchased mail order or on line from another EU country but it seems this has failed.
http://www.guardian.co.uk/eu/story/0,,1955136, 00.html
So....don't buy then or the Tax bill will make them more expensive than if you brought them here.
Bekki