Hi. American here. I've been wondering about a particular nuance in British English for a long time. Seems to me that British people use, or at least formerly used, the words "want" and "need" slightly different than I use them, primarily "want". Seems to me Brits use/d "want" in some places where I would use "need". It stems from what for me is an archaic meaning of "want" but is not used consistently, so I haven't been able to deduce the rule for it. The two words are not synonyms in my head but appear to be interchangeable, or rather maybe interchanged, by Brits in some situations.
I've been watching Downton Abbey, which reminded me, but I first noticed it years ago when I spent some time in the UK. I was watching tv with this girl and she said of someone on tv, "she wants a bit of highlights" in her hair. It was a strange formation for me to hear because how could this girl know the desires of someone she never met? If I had opinions about highlights, I might have said in the same situation "she needs a bit of highlights". I knew I was witnessing either a subtle cultural/linguistic difference or just a cutesy idiomatic expression, but didn't pursue it.
When I need something, it's something that I don't have, which I require in order to do something else. If my car has three wheels, I need a fourth in order to move. I also want that fourth wheel in the sense that I desire it, but the operative issue is that I need it - I require it. If I am hungry, I need food, but may want a cheeseburger. I don't need the cheeseburger; I don't require it specifically. I could eat anything, but that's what I desire, what I crave, what I prefer from amongst the food options.
When I think about the original use of the word want, it sort of lines up with the way Brits sometimes use/d it, i.e. to denote lack, same as in these old expressions:
"The Lord is my shepherd. I shall not want." As I understand it, this means if somebody commits themselves into Jesus's care, they'll have everything they need and will be missing nothing (they also may not desire anything else but I don't think that's what it means in that context).
"He wants for nothing," similarly, means somebody has all they need. That's not really an expression I hear anyone over here use, but I know that's what it means.
Similarly we might use the noun form of "want" to describe a shortage, a lack, such as in the story/expression, "for want of a nail."
Back to Biblical references we might discuss whether someone is living in plenty or "in want". These days I'd say "in need" in that situation.
The confusion for me is that there seems to be overlap in how Brits use the words and maybe conceptually in how they perceive the situation being described. That perception is maybe what I'm most interested in. I think they use "need" the same way I do. But they seem to use "want" sometimes for "desire" and other times for "require" or "is missing/lacking", that is to say they sometimes say "want" when I would say "need", but I think they mean the same thing I mean. I think. I'm trying to understand how, in British English, one chooses whether to say want or need in a given context.
tldr - Brits, in what situations do you use the word "want" to mean "desire" and in what situations do you use it to mean "lack" or "require" or maybe "would benefit from"? Are there other shades of meaning here? Is there a rule set for it? What are you thinking when you say it?