The following is an extract from HM Customs and Excise brochure Flying Abroad? Bringing Back Cigarettes or Alcohol? Don’t be taken for a Smuggler.

You can bring into the UK as much EU duty paid tobacco and alcohol as long as it is for your own use, which includes:
Bringing back goods for your own consumption.
Bringing back gifts for family and friends.

You cannot
Bring back goods for commercial purposes, which includes:
Bringing back goods for payment - even payment in kind.
Bringing back goods for family and friends if they are paying you, giving you the cost price or paying for some or all of your travel costs.
Bringing back goods for re-sale.
If you bring back large quantities of alcohol or tobacco goods and are stopped by Customs you may be asked some basic questions about your trip and purchases and the purposes for which you hold the goods. This particularly applies if you have more than the following amounts:
    3200 cigarettes    200 cigars
    400 cigarillos    3kg of smoking tobacco
    110 litres of beer    90 litres of wine
    10 litres of spirits    20 litres of fortified wine (such as port or sherry)

The above are only guide levels based on European law and Customs do NOT enforce absolute limits.

The questions and checks which the Officer makes could be about:
    The frequency of your travel
    The amount and type of goods purchased
    The reason for purchasing the goods

The EU Countries from which you can bring duty paid goods are: Austria, Belgium, Denmark, Finland, France, Germany, Greece, The Irish Republic, Italy, Luxembourg, The Netherlands, Portugal, Spain (but not the Canary Islands) Sweden, The UK (but not Gibraltar or the Channel Islands)