We would do the scale Extremely unhappy < very unhappy = quite unhappy
It can be both in British English. "Oh yes, 380 runs not out. He was really quite good," being a posh understatement of an exceptional cricketer. The 'quite' may be emphasised. "He really was
quite good."
In general use though, without intonation or context to suggest otherwise, it'll be 'an imcomplete measure of'. But yeah, in it's fullest extent you could have:
Extremely unhappy < very unhappy =
quite unhappy < unhappy < quite unhappy < indifferent < quite happy < happy < very happy =
quite happy < extremely happy
It makes sense when you think of the English unemotional, stiff-upper-lip stereotype. Emotions are downplayed and understated. "Yes, he killed my mother and two children right in front of me. I was quite put out. Needed a stiff brandy to calm the nerves, what?"
Edit : The Brit extended edition:
Extremely unhappy < very unhappy =
quite unhappy < unhappy < nearly unhappy < mostly unhappy < quite unhappy = somewhat unhappy < indifferent < quite happy = somewhat happy < mostly happy < nearly happy < happy < very happy =
quite happy < extremely happy
And "rather happy" which is anything from quite to very. Honestly, you can't tell unless you hear the person say it.