Theoretically, you can bring back an unlimited quantity as long as they are for 'personal use'. ('Personal use' includes genuine gifts - for which you receive nothing in exchange- but otherwise excludes cigarettes brought back for other people).
HMRC have a guideline figure of a maximum of 3200 cigarettes. Under most circumstances, they won't challenge travellers, about the importation of cigarettes, up to this amount. However, you should be aware that you could still face problems if there were circumstances which suggested that the cigarettes weren't for personal use (e.g. if you bring back several different brands).
http://customs.hmrc.gov.uk/channelsPortalWebAp p/channelsPortalWebApp.portal?_nfpb=true&_page Label=pageTravel_ShowContent&id=HMCE_PROD_0102 21&propertyType=document
Chris