As my friend has told me to do in the past, get all of your fags at the beginning of the holiday and proceed to smoke one cigarette from each box. If you get stopped at customs (a pretty big "if") then you can prove they are for personal use because who would want to buy an opened box of fags?! Only downside is that if you are planning to bring them back for a friend you need to bring back all the same brand. My friends actually brought back a huge cardboard box full back between the two of them and they didn't get stopped at all. It's the risk you take. You won't go to prison if you get caught, just means paying bloomin tax on it all.