In answer to your question we see a very mixed bag sailing out of many places including Europe where people are wearing all sorts, but if the cruise sails out of Southampton we see a lot of tuxes, long gowns, cocktail dresses, jackets, ties and not many deviate from that.
On our last cruise this year out of Southampton there were a few people who were not so dressed such as wearing slacks and a polo shirt and you could tell they felt pretty uncomfortable and out of place, probably because they didn’t realise how dressed up the cruise would be sailing out of Southampton.
I don’t think anyone will care what you wear as long as it is smart, it just depends how much you want to fit in with what everyone else will be wearing.