Same rules apply as those for going across on the ferry but the ship might have it's own rules. Check with P & O if anyone should know it is them. Failing that take the cheaper option and give up smoking.... Nei.
You can bring as much as you want from belgium the hard part is uk custems and excise convincing them that it is all for your own use.
You can bring as manyas you like as long as they are for your own use but you can only take 200 bought on boar.