As I understand it, the letter W is vanishingly rare in Spanish - to the extent that all of the articles I could find discussing how it is used, only listed imported English words like waterpolo and whisky.
Are there any natively-Spanish words that use this letter? If not, why is it even there? (I would have thought languages don’t generally have spare letters that they keep in storage just in case they want to import a foreign word)
According to Wikipedia
Which, on the surface of it, means those are not “real” Spanish letters at all in some sense. Loanwords are loanwords of course, whatever letters the Academy “allows” or not, though the use of arbitrary letters from arbitrary alphabets and languages would compromise some principle of having a consistent predictable orthography.
It’s weird that the loanwords didn’t get rendered into a spelling form that just works in Spanish (like football became fútbol). Whisky could have just been huisqui or something.
The analogous feature in English would be accented letters. Ç, å, ø, etc. aren’t English letters, but you’ll still sometimes see foreign words written with them.
Sure, but those letters aren’t included in the English alphabet.
It’s all a continuum. If there were enough loanwords that used ç, say, and those words were used often enough, eventually some sources would start saying that it was part of the English alphabet, just like j and w are now part of the alphabet but didn’t used to be, or how thorn (which I don’t feel like looking up how to type) used to be but now isn’t.
I can only speak for British English, but I don’t think we would. We tend to just ignore the ‘special’ foreign letters and just use something we already have - often more a case of visual similarity than how it sounds - for example jalapeño has pretty much become jalapeno here; pâté has become pate, and so on.
Not saying this is right, but it’s what tends to happen. You’ll still find people who haven’t just mashed it into the English alphabet, but I think they are the minority.
My older daughter, who is fluent in a number of local languages, says that when she and her friends are texting each other in French, German, etc., they don’t bother with the extra marks (umlauts, l’accent aigu, etc.). They just type fast and glean the intended word from context. She says she knows how to long-press the letter on the mobile-screen keyboard to get those accented letters, but nobody does it, and it would be seen as weird if someone bothered.
Which of course invites speculation about the long-term influence of these practices on the formal written form of the languages, but that’s outside the scope of the thread.
The vast majority of Spanish words containing W are imported, as you say. And these countries also import whiskey and read William Shakespeare. Some countries such as Mexico or Puerto Rico are influenced by the United States. Countries like Spain by its European neighbours. It’s not like they had unused letters waiting for an opportunity, more that people were exposed anyway to foreign W’s and no one is confused by keeping the same letter, just as you are not confused if I use the words reëlection, café or mañana (there are better examples). There is perhaps an element of sophistication to using these words, maybe a risk of pomposity. Clutching pearls and crying “But these are foreign!” does not mean they are never used or useful.
We mostly kept it so we could spell “Walter”…
Certainly in English, it tends to transform the pronunciation of the word. Jalapeno (without the ñ) is pronounced by many Brits as ‘jalla-PEE-no’ or ‘jall-AP-uh-no’ (with an initial J sound too), but that goes for words that don’t contain any conspicuously foreign letters - chorizo is often ‘chuh-RITZ-oh’; paella is often ‘pie-ELLA’. To attempt to maintain the original pronunciation will mark you as pretentious in many circles.
Oddly, Habanero (which doesn’t have an ñ and never did) is often pronounced ‘hab-a-nyairo’ here. Go figure.
One difference between English and Spanish, which is relevant to this topic, is that in English, while you can make an educated guess, you cannot expect pronunciation and spelling to correspond in a completely predictable way, so foreign letters and accents are the least of your problems. Whereas in Spanish or Finnish it’s a different story, and loanwords would tend to mess up the tidy orthography.
If these Brits don’t know Spanish, OK, whatever, but if they did know then they would know how to pronounce all those words just by reading them. (I suppose you would also need to know that “h” is silent, etc.)
ps I tend to pronounce Los Angeles with an /x/ 
Like “Loch Angeles”? I like it!
(PS. I know what you meant.)
In theory, yes. In practice it usually goes like this:
“What are the red bits?”
“Chorizo”
“What?”
"Chorizo. Spanish sausage. ’
“You mean churritzo?”
“Um… I mean chorizo”
“It’s pronounced chuh ritz oh”
“No it isn’t”
“Anyway, I thought you were only supposed to use churritzo in pie ella…”
"It’s…no. "
Well, it is a country that bafflingly says “tack-oh,” so… 
It was a coin toss between that and ‘take-oh’ 
I find it funny that Brits say paasst-uh instead of pahstuh, when you all use the American (or Italian?) pasta A sound for everything else.
This would be a lot clearer if I knew how to use those pronunciation symbols…
It’s there just to accommodate gringos like us, and that’s only because we have a lot of tourist money. 
Autocorrect should add the accent marks, at least in French. Idk about German.
This almost certainly isn’t true. pastie, packet, pandemic, parachute, panoply, pancake, pageant, parrot, pastille, pastel, pastoral, passive…