You are allowed 3,200 cigs per person and they are free of tax unless you intend sending them onto someone outwith the EEC. We found Piraeus was one of the cheapest sources in the Med area, but perhaps there might be others even better.
Unless you buy from the duty free shop within the port itself, or on the ship, any baccy or booze that you purchase in shops in Europe will be duty and tax inclusive. It can therefore be imported into the UK without further payment (which I assume is your intention) UNLESS you try to bring in an excessive amount, which is somewhat open to speculation. There are plenty of tobacconists in Piraeus and elsewhere; look for the street kiosks (where bus tickets are also sold).