I thought I would share my experiences with you, in case you find it helpful.
I have cruised 12 times before, Disney, NCL and Royal Caribbean, and I will make some comparisons along the way, and as always it is just my honest opinion.
First of all, Britannia itself:-
Our Cabin (E701) was pretty good and was fairly quiet apart from the normal doors being left to slam. Clean and spacious, probably lacked a couple more drawers. Our steward was great, cleared the mini bar, kept refilling the ice. No complaints and on a par with other ships/lines.