The importer has to show to Customs that they are for their own consumption (there are some technical legal issues from the Hoverspeed vs HM Customs case, but this is practical advice not a legal treatise). Some of the main points Customs will consider is the source of funds, the frequency of travel, whether an attempt was made to cover up the goods (eg under a blanket), the amount the individual smokes (and they will ask you this is a number of different ways to look for inconsistencies) and the attitude when stopped. Anything other than total honesty will be treated with suspicision.