To clarify a few of the points above:
There is no 'duty free' allowance when travelling within the EU. (So Robert551069's post is irrelevant).
You are allowed to bring back a theoretically unlimited quantity of cigarettes from within the EU, as long as local taxes have been paid on them if they're solely for your personal use. (Bona fide gifts to others, where nothing is received in return, count as 'personal use'. Bringing back cigarettes for others where you receive payment, even if you don't make a profit, does not count as 'personal use).
HMRC & UKBA have a 'guideline maximum' quantity of cigarettes which they'll normally not challenge people about as to 'personal use' unless there are unusual circumstances (such as bringing back several different brands). That maximum figure used to 3200 cigarettes but it was reduced last year, to just 800 cigarettes:
http://customs.hmrc.g...tentID=HMCE_CL_001734
This map shows where prices are cheapest:
http://www.the-tma.or...ploads/2011/07/EU.jpg
Spain, Italy and Greece all have struggling economies, so expect to see big increases in tobacco taxation in the near future.
Chris