What is the correct pronunciation of Costa Coffee?
Solution 1:
Surprise, they’re [ˌɔə̯ɫkɚˈɻɛ̝t̚]!
Many, many other actual pronunciations of this word can be found among native speakers of different regional accents. Each one is “correct’ for that accent and speaker, even should this differ from one’s own.
Not only is English a pluricentric language spoken natively by some 400 million people all across the globe, even within a given geographical region, notable pronunciation differences abound between that region’s own numerous internal dialects.
There can therefore be no single “correct” pronunciation of a word for all native speakers everywhere. This holds as true of common words like all and right, mother and daughter, warm and bath, as it does for less common ones.
When you discover evidence that native speakers of differing origins, educations, or occasions all say the same word differently, that does not mean that one accent’s pronunciation for that word is “correct” and the others are “incorrect”. A case in point is that you’ll never get everyone to say the word coffee the same way.
If you don’t believe me, just try to get folks to agree on what the “correct” pronunciation is of words like like route or roof is. Each native speaker will doubtless say that their way is “correct”, but only those with the most severely provincial of viewpoints would dare call alternate pronunciations from other native speakers “incorrect”.
You might imagine that the perceived “owner” of a name has an annointed right to determine what is the “correct” way to say that name, but even here you can sometimes be wrong. That’s because with proper names like those of people, cities, or businesses, this picture becomes even muddier. Anna, Charles, Melbourne, and Oregon are all naturally pronounced differently in some accents than in others. You will never get a non-rhotic speaker to say Charles the way its rhotic owner says it. Or just because Anna herself may happen to use the same vowel there as she uses in apple does not mean that someone of a different accent who always uses the vowel from father for that word is somehow “incorrect”.
Proper names originally from a different language than English are sometimes even more subject to variation, as these are not necessarily only variations by dialect alone. Rather, educated native English speakers with experience in language the word was borrowed may pronounce that word closer to how it is said in the other language. Think of Paris, Berlin, Cairo, Louisville, Lyons, Lima, New Orleans, Santiago, Colombia, Chile.
The particular sound that seems to be concerning you here, the stressed vowel from the first syllable of the word Costa, happens to be a sound that varies dramatically between English’s many regional dialects. This is further complicated by varying impressions of how assimilated the word has been into English from its Romance roots.
It may sound just like the word coaster in some dialects but like the word caster in others. Some may rhyme it with how they say pasta. It might even sound like the last two words in the sentence “That’ll sure cost her”, or rhyme with the first two words in “Lost a penny?”
Each version is “correct” in that accent. That’s why all native speakers’ pronunciations of a word are “correct”.
Solution 2:
In all the (many) bits of the UK I know it's the same "o" as in "coffee", i.e. short. This is also how we pronounce the Spanish "costa" as in "Costa del Sol" or "Costa Rica". A shift to pronouncing it as "coast" seems unlikely in British English, especially with the extended alliteration (for want of a better term) in the coffee brand
Solution 3:
Word pronunciations may vary depending on the broader phrase in which the word appears or on particular local historical factors. In California, where I live, a great many local place names are of Spanish origin. Often the standard native English pronunciations of these names indicate little awareness of Spanish pronunciation preferences—and yet these English pronunciations persist alongside pronunciations of the same or similar words or names that are not local and reflect a much stronger awareness of common Spanish pronunciation.
For example, whereas most (though not all) native English speakers in the San Francisco Bay Area pronounce the country name Costa Rica with a long o (KOAST-uh REEK-uh), and (back in the 1980s) many pronounced the anti-Sandinista Nicaraguan insurgency known as the Contras with a long o ("Koan-truhs"), they almost universally pronounce the East Bay county name Contra Costa with short o's ("KON-truh KOS-tuh").
Likewise, I have never heard a native English speaker in the Bay Area pronounce the last name Allende (in reference to Salvador or Isabel Allende) with an l sound rather than a y sound for the ll (the pronunciation is usually approximately "I-YEN-day"). Yet almost all native English speakers here pronounce the city name Vallejo with an l sound ("Vuh-LAY-ho"). (Note that they do use the Spanish-preferred h sound for the j.)
Further, most native English speakers here pronounce the familiar name of the artist Raphael in three syllables (approximately "Rof-I-EL" or "Raf-I-EL") but they pronounce the second word in the city name San Rafael in two syllables (approximately "Ruh-FEL").
The conclusion I draw from these (and similar) inconsistencies is that pronunciation is largely an aurally received phenomenon, and it is received from different sources at different times—rather than being formulated systematically and cross-checked consistently on the basis of identical or similar spelling. The name Costa Coffee is not one that I've encountered previously, so when I first met it in print (just now), I had no idea whether people who are familiar with it generally pronounce it with a long o or a short one. But the fact that people may be inclined to pronounce it different ways in different places doesn't surprise me at all.