A quick Google gave me this:
https://www.studying-in-uk.org/best-veterinary-schools-in-the-uk/
I am American, but it's difficult to believe that vets, especially farm animal vets,
wouldn't necropsy animals that died of non-obvious causes - especially in (relatively) large quantities. You're the country that got hit by Mad Cow disease. The yearly revenue for poultry farms in the UK in 2019 was something like four billion Euroes. Nobody wants to risk... I don't know, Crazy Duck Syndrome or something destroying the industry. So there's a decent chance a mass death, even of ducklings, will be taken somewhat seriously by scientists.
I'm just saying go ahead and reach out to somebody now. Describe the situation. You might get a free inspection/check-up out of it. Best case it prevents a recurrence in the next hatching. Worst case, they tell you to "sod off." But the case will likely stick in their minds and if something similar happens, to you or someone else, that memory could give them the jump on finding the cause and cure.