Well, just for the record on my QM2 crossing (was yours a crossing, Eric?) both of the balls were very well attended. Plus I would have to say it was one of the most formal experiences I have ever had on a cruise ship.
I would say we have about 60% Brits on that cruise, by the way. And everytime I have been on Cunard it has been in the Princess or Queens Grill, so I dont really know Britannia.
Lets just say that if I showed up on the cruise I was on without my tux I would have been just a little upset that I had failed to bring it.
I would say the formal nights on Crystal came close for ther number of tuxes, back in 2008.
But of course EVERY cruise is different - I was on the inaugural cruise of Liberty of the Seas and it had a lot of Crown & Anchor members, and I was astounded to see that at least 80%+ of the men had on tuxes.
So in the long wrong it is more about dressing formally if you personally it enjoy rather than worrying so much about what other people will think. I think it shows that you are an experienced cruiser to do so, and that you have the class to show up in formal dress even knowing maybe not everyone will agree. Even if I hear a ship is pretty informal, and I decide I want to go formal anyway - then I see no problem with that.