Why aren't people from New England called New Englishmen?

IIRC, the TV programme was renamed, but the film was ‘ninja’ on its original release.

Does “posh” mean rich, or stuck-up/snooty, or…?

That’s a tricky one. You might well get a fair bit of traction if you post that as a GQ, although I’m not sure that there is any kind of factual answer. Perhaps an IMHO topic? Certainly there seem to be enough UK Dopers to give you some sort of flavour of the current state of the British class system ie it’s an utterly chaotic mess that no two people will agree about.

That’s just what chav scum like you would say!

Regarding the formation of demonyms as in the OP, I think the easiest answer is that we’re speaking English. English (and New English) pretty much ignores standard constructions for its demonyms. Someone from Massachussetts is a “Bay Stater”. A resident of Glasgow (UK) is a Glaswegian, and a resident of Glasgow (Delaware, US) has probably never heard the term and would sock you on the jaw if you called him one.

IANAB, but I’ve read it has to do with Guy Fawkes, a conspirator in the plot to blow up Parliament on November 5, 1605 while the King was addressing both houses. The breakup of the Gunpowder Plot is remembered each year on Guy Fawkes Night (AKA Fireworks Night or Bonfire Night), during which effigies of Guy Fawkes are burned - these effigies are called “guys.”

Given this, the term “guy” has historically not been seen as a compliment in Britain. I suspect that the original meaning has faded - that it has become more neutral in Britain, possibly because of how it’s used in the U.S.

I’ve heard that one on the boards too but just had to look it up!

Hmmm. It never crossed my mind that ‘guy’ would be used as a disparaging term. I’m confident in saying that such usage died out well before snobs started objecting to the use of the Americanism.