[traceroute66]

> Both “French” and “British” are acceptable demonyms

No they are not.

The Oxford English Dictionary, for example makes it quite clear re. 'French':

    "With plural agreement, and frequently with 'the' French people regarded collectively ..."

I draw your attention to the first three words ... "with plural agreement".

It is explicitly telling you that "French" is a collective plural noun and hence cannot be used as a singular countable noun.

[woodruffw]

I think we’re past OED being a normative arbiter of what does or doesn’t pass for acceptable English usage.

[troad]

Precisely so: the OED's role is descriptive, in that it is to describe English as it is used, not how it ought to be used. It provides evidence of a grammatical rule - it is not the rule itself.

You can speak however you like, there is no language police, but the fact is the average English speaker will perceive certain constructs to be grammatically incorrect. "He eat", "I driving", "a French" etc.