UK passport application - 3rd country passport lost

[obormot]

When one applies for first UK passport, they require to send all passports of other countries.
We seem to have misplaced one of two passports of my younger kid (age 11) - the non-EU one (he is already double national - from France and from another, non-EU, country). We still have his French passport.
Can we apply just sending his French passport? Should we mention the fact that he has yet another nationality but we are not in possession of his passport for that country? Would UK authorities require us to get a replacement for that passport before giving him A UK one?
That would seem excessive, but who knows..
(To get a new passport would be a long and painful process, which could take more then 6 months to a year; and we do not ever use that passport anyway - it would be only needed to go to that country)
We did not submit a copy of that passport for naturalisation application.

[alterhase58]

HMPO are not concerned with whether applicant is a dual naational. Reason they request passports is to make sure that the same name is used on all travel documents. If you have lost a passport then you can't provide it, and not need to declare the fact, or to get a new one.

[obormot]

Thanks.
That was my reasoning - assume I have 2 citizenships - then they might want to make sure that my name is the same in documents for both countries (so they might insist on me providing a proof of how my name is written for both countries - be it a passport or something else).
Otherwise i do not see the point of asking me to send all passports - say, I currently do not have passport, so they do not see it, but later I can get it and if the name is different then they did not do their check..
But probably I am overthinking it..