Cunard has always set the standards high and the rest simply follows. Congratulations on making that choice as the trans-Atlantic is one of the best way to relax & unwind, away from the rest of the world for 6 days. I believe there is 3 formal night, 2 informal & smart casual.She is a fantastic ship, has 4 major elevators (A=forward - D=aft/back) deck 2,3 & 7 is where you shall find play,food, drink and Canyon Ranch Spa, endulge & bon voyag.
I agree with the last answer, Cunard has higher standards than P&O - or more accurately they have kept the standards that P&0 used to have before it was aimed at the mass market by its US owners. Cunard tranatlantic is a different experience - with no ports of call - and thus its US owners have focused on providing an on board elegance, wonderful things to keep one entertained and more attracts people who do not find it an imposition to dress for dinner. However it is not at all starched shirt or stuffy but would be seen as such only by those who want to come directly to dinner from the poolside, complete with a pint of lager in their hand.
You are in for a wonderful time,with outstanding service, food and entertainment. For those that do however like the P&O formula they do have one bar that is a pub, with pub entertainments but this is easy to avoid if you want to.