According to the Government's own web-site (see related link) there is (apparently) no restriction. HOWEVER you must be able to satisfy customs that the amount you're bringing back is for PERSONAL use.
If you tried to bring back ten kilos of tobacco, you'd have a hard time convincing customs you're a 'heavy smoker' - so - just be sensible with the quantity.
Anyway - remember - you can always split it amongst your travelling companions (provided they are over 18 !)
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