It would seam you legally can buy whole cured leaves without duty, about £20 a kilo. They state that you must contact HMRC if you are going to slice it up for smoking and they are just supplying it for other purposes eg insectercide and medicinal BUT on the same page they offer to sell you the equipment to slice it up sub 1.5mm for cigarettes.....although I wouldn't risk it it would seam that the evidence they would need to prosecute you would boil down to cured leaf, processed cigarettet rag and specialised equipment for chopping it up.
its a minefield so if I do grow any it will be only if I can prove that it's just for insecticide and ornament.
ATB
Tom