There is no weight limit on luggage if you are sailing from Southampton. Normal duty free rules apply i.e 1 litre spirits and 200 cigs if purchased outside the EU.
There is a limit that ships allow but normally you would get nowhere near it. The luggage allowance is something that is exercised by airlines and they all seem to differ in some way or another. So if you are flying go on to the web site of the airline concerned and there it will be fully explained. If you are going in and out by sea then don't worry about it too much but P&O's brochure or web site will explain fully. If you are travelling to and from EU countries there is no "Duty Free" any longer and providing you buy your goods in their shops then the only criteria is whether it is for your own use. For countries outside of the EU and for product purchased on board, I think, then 1 Litre of spirits and 200 cigarettes apply. Hope that's helpful....Falan Dow.
Beware purchase aboard ship for inport into the uk are treated by hm customs as non eu purchases.