Headquarters is not always plural, nor always singular, as noted in your link. Among responses at that link, "Loob" in England writes:
... if I were talking about a military HQ like the one I used to work in, I would definitely use "is".
But for a company headquarters (= the HQ of a single company), I'm pretty sure I use both "is" and "are". I was putting this down to that free-and-easy way we Brits have with collective nouns ...
Likewise, I imagined that headquarters is would be found more often in British usage than American. However, that notion is not borne out by American- and British- corpus ngrams for headquarters are,headquarters is :
As can be seen, headquarters are is more predominant than headquarters is, in both corpuses, and is more markedly so in the British English corpus than in the American English corpus. (Note, there are numerous confounding cases that ngrams does not sort out, e.g. headquarters appearing at the end of a sentence or phrase, and is or are starting another; or reference to multiple headquarters establishments. However, I think those are probably a small enough fraction of instances that the general idea holds.)