Truly believe that service in Britannia is in direct correlation with your particular server.
Our waiter was not terrible - but certainly not up to the Cunard standard we had experienced in past cruises. Wouldn't have been surprised to find out that he was pulled from a Carnival ship - only base that on one experience with Carnival.
Placement of your table in Britannia might also have something to do with it. Haven't heard a bad review from anyone who was in close proximity of Captain's table on first level.
Would certainly sail on QM2 again, but there are glitches to be worked out. We had a wonderful time.