Well you can't beat information from an on the spot reporter.
Watchet station's platform has been changed since the last time a HST▸ visited the WSR.
Now we know it isn't "the" Flying Scotsman" but is it "a" HST or "an" HST.
'H' is not a vowel, and as far as I know you only use 'an' if the following word begins with a vowel. So, a class 180 is a HST and a class 158 is an Express Sprinter (also known as an Alphaline). An issue however is that "an one-five-eight" sounds wrong, "a one-five-eight" sounds better but as far as I know is technically incorrect (since 'o' is a vowel and 'one' starts with the letter 'o').
I find that a surprising opinion from any Welshman, especially one with a forum name that includes a 'y' that's 100% a vowel. Both 'y' and 'w' are primarily vowels in Welsh, and I reckon 'y' in English is more often a vowel* (as in 'early') than a consonant (as in 'yet'). 'W' as a vowel only happens with imports from Welsh, mostly names, but at least one word: 'cwm'.
Basically all this stuff about a letter being either a vowel or a consonant all the time is rubbish. It's all a matter of pronunciation, and in the spoken language it's your own speech that counts. For written English you can say that there is a notional standard pronunciation that determines such things, but it's very hard to pin down what that is. After all, spelling doesn't represent any standard pronunciation, does it?
So, for just the use of 'a' and 'an', it's the sound at the start of what follows that matters. For this case a straight vowel/consonant division is OK (for other things you need to split consonants into stops and continuant). So yes, long initial 'u' (often) and 'o' (rarely) can start with consonantal 'y' and 'w' sounds, and so be preceded with 'a'.
But the example that started this wasn't about words, it was about letter clusters, including abbreviations. (Perhaps that ought to be broadened to "symbol clusters", but that's not needed here.) So it's not the sound of a word spelled with a letter that matters, it's the sound of the name of the letter.
Now these names are rather odd, in that we all know them in spoken form, but have only a semi-official written form and spelling at best. Mostly we borrow a word with the same sound (e.g. 'jay' or 'see'), though a lot of them exist as words derived from the letter ('eff' or 'em'). And eight of the consonants have names that start with a vowel.
As for 'h', it's a special case, as it has two forms: the standard 'aitch' - which does start with a vowel - and the dialect form 'haitch'. This is common in some areas, and in Northern Ireland is a marker of community identity.
But words starting with 'h' contain many special cases as well, as noted in my post above. It's not just a matter of the uneducated "dropping aitches"; there are loan words from French and older English pronunciation patterns. But it could be worse - after all, French has two kinds of 'h', distinguishable by pronunciation, but both silent.
And for one more puzzle, I know of one word in English that has only an abbreviated spelling, and some full spellings imitating the speech of a place or class, but no full-length spelling in standard English.
*Some of these sounds may be called semi-vowels or semi-consonants, but I don't think that's helpful here.
