Why the "ones" in this sentence "French students have to work harder than British ones." is incorrect whilst in another is correct?

It seems true to me that in a sentence such as

• "There were people strolling about; the English ones were dressed warmly.",

"the English ones" doesn't seem to correspond quite to what one would say in English and something as follows could be more usual.

• There were people strolling about; some were English and dressed warmly.

It is possible that the use of the pronoun acquires some legitimacy when the context makes clear that the type of person considered is found in the group under discussion.

• There were persons of all nationalities in the group; the English (ones) had arrived yesterday.

However, this is mere conjecture; independently of a consideration of the incidence that this principle could have, there exists the evidence that the rule, as simply stated, cannot be a solid one; in recent enough publications that can be considered serious, the phrase "British ones" where "ones" stands for persons is often used (all types of English). Eight examples are found below, four of them come from the corpus of British English.

(ref. 1, 2018) I've always taken my bearings from American poets more than British ones though.

(ref. 2, 2013) French Muslims who tend to be less pious than British ones identify more with France than their British counterparts.

(ref. 3, 1948) […] easier for a highbrow critic to overlook the absence of refinement in American film makers than in British ones.

(ref. 4, 1997) While German analysts are rarely in doubt about the ' industrial sabotage ' element of dismantling policy , the British ones seldom accept this as the motive for dismantling.

(ref. 5, 1960) However , I should just as well confess that I feel more akin to the practical breeders , and I am proud that I may reckon some of the distinguished British ones among my good friends.

(ref. 6, 1975) […] and among colonial administrators. Most of the British ones came from public schools where trade was regarded with distaste .

(ref. 7, 1965) modern art is now so international that artists other than British ones have inevitably

(ref. 8, 1986) who had scowled at the idea of paying American writers ten percent royalties when he could get British ones for five or six , cut his outgoing royalty bill even further by once more taking a music credit for himself and a book credit for

From the directives on using the pronoun "one" as given by Practical English Usage (Michael Swan, second edition 1995), the restrictions on the use of "one" are those.

1 It is not used for uncountable nouns
If you haven't got fresh cream I'll take tinned (cream).
2 It is unusual to use "one" for abstract nouns
The Dutch grammatical system is very similar to the English system.
More natural than …to the English one
3 One(s) is not generally used after noun modifiers, except those which refer to materials
Do you need coffee (cups) or tea cups? (NOT…or tea ones)
BUT We can lend you plastic chairs or metal ones.

Here are a few more examples from the British English corpus for "English ones".

(1, 2012) This suggests that these lords married the daughters of fellow Irish, English and Scottish peers (53 per cent), with Catholic peers preferring Irish brides and Protestant peers favouring English ones.

(2, 1984) I must say , some of the immigrant children where I am seem a better class of child than the English ones .

(3, 1923) The Swedish ambassador has remonstrated with the English ones on behalf of the Chancellor Oxestern because their king only showed an extract from the articles of the alliance between France and England

(4, 1970) Those for Sweden have reached this city and are now meeting the English ones to discuss the question

(5, 1994) ... a member of which invaded Ireland in the 12th century along with Strongbow ) ; ( 33 ) LBJ ( a surprisingly strong German showing , plus Huguenot , Scottish , Swiss and Irish elements , also presumably English ones judging by surnames ) ...

(6, 1908) American commercial travellers come round and push , and bounce , and promise everything that is asked , and supply it too ; whilst English ones go about in a leisurely fashion , and often only undertake to furnish out - of - date things

There are, similarly, plenty of cases for "young ones", "old ones", "dead ones", "tall ones", etc.

(dead ones, Nikolai Gogol's Dead souls) Leave the live ones in peace; it's the dead ones I'm after." "Really, I'm rather hesitant. I could lose money entering into such a transaction for the first time. . . . Maybe you're fooling me, my good sir, and they're really worth more."