I suspect most of us fly different classes (but always hope to bag an UC one whenever we can afford or get a deal on one) so I suppose the other part of the etiquette is to try not to look too smug as you turn left and keep the dancing / skipping towards the front of the aircraft to a reasonable level of decorum.... then grab some bubbly & enjoy / recline!
People are definitely more casually dressed on return flights from beach holidays, even in UC, so there is nothing wrong in dressing for comfort and then (as Bretty suggested) maybe having a change of clothes and something other than beach shoes for landing back in cooler climes. Sensible rule is imagine how you would feel if someone sat close to you dressed as you intend to dress .... shirt & tie unnecessary but something like a clean t-shirt rather than a vest and clean shorts rather than a swimming costume & preferably without 3kg of beach debris stuck all over yourselves

Hope you have a great holiday, I'm sure you'll enjoy the UC experience - don't forget to post your trip reports as we do like to hear experiences (good or bad).