When travelling to the UK from another EU country (excluding a few of the 'new' member states), including Greece, you may bring back an unlimited quantity of cigarettes provided that they're for your personal use. (This can include genuine gifts to other people but not cigarettes for which you will be paid).
In order to deter the smuggling of cigarettes for commercial gain, HMRC have issued a guideline for what they will normally regard as a 'reasonable' quantity for personal use. The figure is 3200 cigarettes (i.e. 16 packs of 200). If you bring back more than this you might face questioning from customs officials.
Several people on AB have previously pointed out that, even if you don't exceed the guideline figure of 3200 cigarettes, it might be unwise to bring back several different brands of cigarettes as HM Customs might still take the view that it's unlikely that these would all be for personal use.
Chris