Greece is in the EU. When returning from any EU country (excluding a few of the newer member states, which aren't relevant here), you can theoretically bring back an unlimited quantity of cigarettes, as long as they're for your personal use.
HMRC have produced guidelines which indicate the maximum quantities of tobacco & alcohol products that they'll normally allow people to import, without challenging travellers as to whether the imports are for personal use or not. The relevant figure for cigarettes is 3200.
It should be noted, however, that HMRC have the right to confiscate the entire quantity of cigarettes (irrespective of whether the 3200 'limit' has been exceeded or not), if they have reason to believe that any of them are being imported for 'commercial' purposes. In particular, they're likely to be suspicious if you bring back several different brands.
Chris