Never stand on the left side of the escalator. Stand on the right, pass on the left.
Never try to push ahead in queue (Am: line). This makes the locals very angry.
Never say anything derogatory about the King. The British themselves may think one way or another about him, but he is a national symbol. Keep your mouth shut as a foreigner.
Do not rent a car, at least for moving around the city. The traffic is horrendous, there is no parking space and on top of that the traffic on the left... Londoners all take the tube except the royal family.
Do not talk to strangers and make eye contact with other tube passengers. If you must speak, the two permissible topics are the weather and the stranger’s dog.
Don't be late.
Do not share your opinion about BREXIT. You never know what your chat partner will think about it. The conversation can get heated.
Don't cross the street without looking both ways. The cars drive in an unfamiliar direction and it is hard to get used to at first and it is easy to get lost. Better safe than sorry.*
Don't enter the train until the people have got off.
Don't call the Westminster Palace Northern tower Big Ben. Big Ben is the bell inside it. The name of the tower itself is Elizabeth Tower.
Don't stare at people. Locals value privacy. If someone should sing the Zanzibar national anthem while standing on one leg in the street, that is their right. Take a look and walk on.
Don't ask personal questions. You can ask questions about another person's service, education, political views, religion, etc. only to a close person, not to someone you are meeting for the first time.
Don't forget to use the expressions sorry and thank you. It is impossible to overdo these.**
* I recommend looking both ways so that you don't get hit by a car when you get back to your home country.
** For example, if you want to order coffee in the park cafe. Of course, you can shout: "One latte." You'll get it. But it would be nice: "Good morning. Nice weather today. Sorry, can I have a caffè latte, please? Thank you."
By the way, it is better instead of "latte" say "caffè latte", the person behind the counter may be an Italian. In Italian latte means milk. :-)