Why is it so common to be cold/unfriendly/antisocial in the UK?
My guess is that you have gained that experience in London, or another large city. This is a common attitude to London, Paris, New York, Zurich or Rome.
Where I was brought up (in deepest rural Derbyshire), it was normal to say hello to everyone you pass on the village, we walked in and out of our neighbours' (unlocked) houses, and conversations were struck up at the slightest pretext.
Big cities are anathema to casual sociability.
The British are more reserved in general than some nationalities, but they are not unusually so for people living in a northern clime.
I don't know where you are originally from, but I guess I somewhat understand your question - I moved to London at the start of this year, and previously lived in Edinburgh. In Edinburgh I always felt as though people were quite friendly, though of course, London is very different.
I'm Australian. In Australia, we do tend to be "actively polite". I actually lived in Japan also and I think they too are more actively polite than here (in the sense that they must welcome you into a shop rather actively, they will ask you if you need help promptly, etc). Certainly, the UK is reserved in many ways, but I do think that people are genuine once you perforate that initial apparent 'coldness' (which is usually just reservedness, I think).
For me the most annoying thing is customer service. Here everyone is eager to get rid of you and not help you, not say hello or speak to you in stores/anywhere. In Australia I think we have a very good balance between being friendly and not overstepping the mark.
But the reality is that I can't expect the UK to be like it is back home. I have to just remember that it might take more time here to get things done, or to form a relationship. I am hopeful though that when and if I do form new friendships with people here, they will be the kinds of ones that last.