It is not a trait I am fond of.   I lived in England for a while and I find it very off-putting.  I like emotion, tears, hugs, warmth, not coldness or people who are aloof.   But that's just me, everyone is different.

It can be a good thing, like being cool under pressure. But it can also been seen as the quality of someone cold and uncaring.

I don't think that is a good phenomenon, we human beings are social aniamls, we live in the group.

But in recently years, I think people become cold and stoic, they rarely show their feelings in public, even in their own family, they become more and more emotional reserved. not just only British, but also it is happening in China. I commute in rush hours by subway, I meet hundreds and hundreds people on my way, everyone is stiff upper lip. we don't smile and we don't talk.

I've never been in England, but I've heard that people from England are extremely strict, I agree that they prefer to keep a distance, but not to the extent of being aloof. I reckon that everyone like to love and being loved by others, because human being are naturally sociable and no one would like to live in a solitary life. 

Talking about relationships, I think men are not supposed to keep the stiff upper lip, because they say that men who have to do the first step and show their feelings.