You might be excited if you plan to visit London first time. In my opinion, London is the best place to be able to see kindly and polite people. But, it might depend on your experiences. If you get conned by a purse-snatcher at the London streets, your senses about London will be terrible :) It is not common, but it had better pay attention your pockets and belongings. Don't forget to you visit a metropolitan city which its people are come from all around the world, from different cultures.
Get an Oyster Card or use your credit card!
If you are a rich person, this point doesn't concern you :) You can take a Black Cab to your hotel from Airport. But, this situation actually doesn't change the reality that London is a really expensive city if you don't get salary as a pound. Anyway.
You have to buy an Oyster card to use The London Underground, Buses and other transportation vehicles. There are machines and kiosks at each station to do this. And it is also possible to use your contactless credit card or your iPhone (Apple Pay) to use them. Both methods charge you the same price. You can learn recent charges from tfl.gov.uk
To access a train, you have to touch yellow circles with your card in the ticket barriers. After your journey is finished, you must touch there again. If you don't touch, the system won't understand where did you go from where and will charge you at full price.
(Note: Some airports are directly connected to city centre via underground services but don't forget you can't do this when coming from some airports like Stansted. Don't be upset, National Rail services are waiting for you to take the city from there...)
Get used to saying "Thank you" and "Sorry"
I said you, you will see polite people that always say you "Thank You" and "Sorry". At the supermarkets, restaurants, stores, undergrounds and pavements... Most of the people that I have seen do this in Zone 1. If you don't say this, people can judge you. Be involve to them on and be polite to other people as required by humanity.
Be ready to "Take Away" culture
The countries I have been, "Take Away Culture" wasn't common but London has so much. What is "take away"? It is an eating way for your meal due to London is a crowded city. The restaurants or fast-food chains offer two type price to their customers as "Eat In" and "Take Away" price. If you prefer to eat your meal in the restaurant, you have to pay more money. In the other case, you can take you can take your meal and eat it outside with pay discounted price.
Always keep left!
Welcome to Britannia, you should drive on the left, you should walk on the left, you should stand on the left, left, left...
For The Europeans, The Asians, The Americans or all people who keep on the right in their countries; this is actually a hard thing to get used to it. You have to pay attention this rule at London. When you want to pass through a pedestrian cross, you should look left firstly! You have to stand on the left when you use escalators.
Mind the gap!
The last thing that you should know about, you must mind the gap! Where? Of course in the undergrounds. Because there are critical gaps between trains and platforms. Get used to hearing announcements as "Mind the gap". This announcement will cause an addiction to you after a while. Also, you can see some t-shirts and magnets which has "Mind the Gap" motto :)
Lastly, Have a nice trip to London! You will really get enjoy, I guarantee you. If you have any question, you can send a message to me.