I'm sure the bit about standing up for a woman has been written in many etiquette guides over the years. So whether code or rule, it is not unwritten. Interestingly, I think usually when someone of either sex arrives at a table everyone else tends to stand to greet them. As for every night being a formal night, that just seems what some people would like to believe.
I’m sure there are unwritten rules, usually to do with politeness. Smokers tend to try and keep their smoke away from those they are with, and, while there are signs on some escalators on the Underground saying stand on the right, Londoners do it anyway, wherever they are. Another unwritten rule seems to be that Cunard ships always leave Southampton a bit late. 🙂