Understanding British English - understatement, passive-aggression etc

I would do this differently, your approach is wrong.

I don't want to follow your advice and want a different solution.

I value your input, but can we explore some alternatives?

You are selfish and your happiness is your only focus. I am not happy.

Well, if you're happy with it, that’s all that matters.

The lack of adjective before choice implies it is a terrible or questionable choice.

Again, by not saying that you like the new hairstyle you are saying it's terrible.

I see you're trying something different with your hair.

Again "one way" is not a good way. By not explicitly saying that it's a good way, we are saying it is a bad way.

Again, not saying it is good implies it is bad. You're into something strange or unusual.

If that’s what you’re into. (a new hobby or behaviour)

Sarcasm. Speaker is not happy for the person.

By saying "if" the speaker doesn't take responsibility and is saying it is the fault of the listener.

We're British: Usually to avoid affection or conversations about feelings.

No, we can't do/talk about that, we're British.

A major disaster has happened. Usually in a work or travel context.