I am not an old school person. I believe that manners and dress codes are to be taken more loosely than in the past, but since I am in London I have become a convertite. If there isn’t a dress code, people look rather misplaced and the German word "Fremdschämen" comes to my mind quite frequently.
Not so long ago, I was invited at a lunch where all the guests arrived on time. However, one person did run 40 minutes late. On their arrival, they spotted their host, continued (visibly!) talking on the phone, and then took a seat at the table whispering ‘Hi’ without apologizing or shaking hands. You could just feel the embarrassment. Of the host. And everybody else.
There are people who ignore the rules because they believe they are above them. A Russian fellow once explained to me: “If you have to obey rules and stick to manners in my country, you’re simply not rich enough." Then there are the ones who just don’t know them and think it’s pretty normal to act the way they do (though watching a very well dressed young woman in a fancy restaurant using knife and fork like a conductor in a classic concert is always amusing). You’ll have to watch out for the third group: standing out by being perfectly fine.
Therefore follow three simple rules: dress according to the occasion, it is better to be over than underdressed, and manners matter.