We will also be sailing out of Southampton and my DH, also a Brit, will be taking his “dress up” clothes. We never cruise into any US ports anymore, as we prefer Europe and the Mediterranean. I think the difference is primarily American lifestyle and culture. “Casual dress” in the US is much different than “casual dress” in Europe. Even here in Canada, my sons wouldn’t even think about wearing shorts and t shirts to a nice restaurant. When we cruise, my DH wears a nice jacket, with or without a tie, and nice dress pants on casual evenings and dresses up for formal evenings. This is the norm for people on the cruises we take, so the “new casual” is not really anything we observe.