But the question states "Most populous European country not using the Euro", not "Most populous Euro member country not using the Euro". So UK is a correct answer. Actually to the quiz maker, if you take it by the letter of the law the answer should include Turkey (use Lira, twice as populous as Poland).
https://worldpopulationreview.com/country-rankings/total-fertility-rate
https://notesfrompoland.com/2021/04/08/baby-doom-poland-records-one-of-eus-lowest-fertility-rates/