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The other lad pointing out that it is in fact a sign of courtesy is on point, it's how people cope. But it is worth taking this a step further and pointing out that people generally take on the manner of the environment they are exposed to. It's not so much that Londoners, Westerners or white people are innately colder than other groups, rather we have crafted a terrible environment for ourselves wherein we have very little personal space.
Adam Smith wrote that the division of labour causes people to become 'as stupid and ignorant as it is possible for a human creature to become', just by virtue of having to repeat the same actions every day to earn a living. Similarly I think the increasing, unpredictable demands on our time, less accessible living space, as well as the monetisation of everything we can stick a value on, will make us as asocial, selfish and defensive as it is possible to be.
You could be forgiven for thinking this is a racial or class thing, but you will notice that people of any background are forced to adopt this manner after living a certain length of time in that environment. Not because they want to, or because it's their character, but rather just to cope. There are exceptions among groups who retain a sense of community, but this is usually in spite of UK culture rather than because of it.
The building block of our society is supposed to be the family, but even these bonds are breaking down due to precarious work patterns (where working class women in particular suffer). I can't speak for everyone here, but anecdotally there's more competition between family than cooperation. I haven't even met all of my cousins, let alone built any meaningful relationships with extended family; immediate family is made to be difficult enough.