The pedantic answer to your question is 'none' since 'duty free' allowances were abolished years ago.
However, I suspect that what you really mean is 'duty paid' allowances. You're entitled to bring back an unlimited quantity of cigarettes and alcohol, on which you've paid local taxes, as long as they are for your personal use. (The definition of 'personal use' includes genuine gifts for other people but excludes any imports for which you'll receive anything in exchange).
In order to clarify the position regarding the amounts that they regard as reasonable amounts for personal use, HMRC have issued a guideline maximum figure of 3200 cigarettes. They won't normally challenge travellers about quantities up to this level unless they've got specific reason to do so. (e.g. they might start asking questions if they spot that someone taking business trips is importing 3200 cigarettes every week. Similarly, they might ask questions about 'personal use' if someone brings back several different brands).
The guideline maximums for other tobacco products are 200 cigars, 400 cigarillos and 3Kg tobacco. There are also guideline maximums for alcohol but, if you're travelling by plane, you'll reach your baggage allowance well before you attain these maximums. See here for details:
http://customs.hmrc.gov.uk/channelsPortalWebAp p/channelsPortalWebApp.portal?_nfpb=true&_page Label=pageTravel_ShowContent&id=HMCE_PROD_0102 21&propertyType=document
Chris