You can take as much luggage as your cabin space allows...all free. This is a frequent point in Cunard's advertisement of transatlantic voyages. And on dear old QE2 you could have taken even more, as they would have kept it in the hold :).
Not a problem regarding the weight bring back as much as can carry, only problem is hm customs in southampton they can and do tax you on clothing you should check to see what your allowance is.
Cruise Ships do not charge excess baggage.....but don't give 'em any ideas!