Excellent advice from Selbourne.
I would just add that dress code on Britannia is perhaps different to what you may be used to.
Without starting a dress code argument (again !) on formal nights in the MDR you will be expected to wear at least a jacket and tie, with the majority of men wearing suits or dinner jackets, and no shorts in public areas in the evening.
One other difference - tips are included in your fare. Again, without wanting to open a can of worms, many of us may tip our steward at the end of the cruise, but there is no need to tip waiters and bar staff on a constant basis.