Since you asked about the legalities, I'll point out that supplying them to your sister would have been illegal anyway, so getting rid of them down the pub is no different. (You're only allowed to import tobacco products into the UK, without paying excise duty, if they're for your personal use. The definition of 'personal use' includes bona fide gifts to others, where you receive nothing in return, but providing them 'at cost price' - as you were planning to do - is illegal).