I sailed the QE2 a couple of years ago with my partner. It is a formal ship and I think you will find the majority of Caronia class passengers wore tuxedos on formal nights and jacket and tie other nights. Don't worry a dark suit will do you just fine. The world cruises always attract the most interesting passengers. You will have a wonderful time.
We had a C class cabin(4006) but were assigned the Mauratania restaurant. I loved the look of the Caronia, it was much more elegant and has a real old ocean liner feel to it.
Enjoy the cruise.