For me it had to be the sheer number of insincere expressions the British use:
They are not insincere expressions. Insincere suggests fake or artificial, but they are polite social lubricants. You are misinterpreting them.
For instance, “How are you?” or “Alright?” at the start of a conversation or as a greeting is understood not to be a direct question, requiring a full response. It’s an introduction to the conversation, which might develop into one in which you are indeed asked about your health or state of mind.
(It’s no different to the French “ça va?” Not a purely British thing at all.)
Other phrases that you are judging to be insincere (i.e. “fake”) are designed to be considerate of the other person.
“What do you think of my new haircut?”
“It’s shit!” might be sincere, but it’s not very kind.
“It’s interesting,” is a way of softening the blow.
Of course, these uses are culturally specific. If you are used to a more blunt/straightforward style of discourse (e.g. German or Dutch cultures) then this use can feel dishonest. But if you are used to a more mannered culture (e.g. English or Japanese) then the blunter style of discourse can seem rude and cruel out of context.
Context is what’s missing from so many of the posts on this thread. Many of the things that seem odd about Britain seem less odd when you understand the context.