Ref

Are there really less than 30 Critically endangered mammal species in all UK collections? This surprises me.

@Swampy This was based on the zoos I'd actually visited, so there are probably a few more, but not many. The screenshot is just an example to maybe help Zooreviewsuk task.

That's all of them in Europe according to Zootierliste

1. Addax (Addax nasomaculatus)
2. Addra gazelle (Western dama) (Mhor) (Nanger dama mhorr (Syn.: Gazella dama mhorr))
3. Amur Leopard (Far Eastern Leopard) (Panthera pardus orientalis (Syn.: Panthera pardus amurensis))
4. Arabian leopard (South Arabian Leopard)* (Panthera pardus nimr*)
5. Bawean deer (Axis kuhlii (Syn.: Hyelaphus kuhlii))
6. Black and white ruffed lemur (Varecia variegata variegata)
7. Blue-eyed black lemur (Sclater's black lemur) (Eulemur flavifrons (Syn.: Eulemur macaco flavifrons))
8. Bornean orangutan (Pongo pygmaeus (Syn.: Pongo pygmaeus pygmaeus))
9. Caucasus Leopard* (Panthera pardus ciscaucasica (Syn.: Panthera pardus transcaucasica)*)
10. Chilean chinchilla (Long-tailed chinchilla) (Common chinchilla) (Chinchilla lanigera (Syn.: Chinchilla velligera))
11. Chinese pangolin (Manis pentadactyla)
12. Colombian black spider monkey (Ateles fusciceps rufiventris (Syn.: Ateles fusciceps robustus))
13. Colombian brown spider monkey (Ateles hybridus (Syn.: Ateles belzebuth hybridus))
14. Comoro flying fox (Livingstone's flying fox)* (Pteropus livingstonii*)
15. Cotton-top tamarin (White-plumed tamarin) (Pinche) (Saguinus oedipus (Syn.: Oedipomidas oedipus) (Syn.: Saguinus oedipus oedipus))
16. Crested black macaque (Sulawesi black macaque) (Macaca nigra (Syn.: Cynopithecus niger))
17. Eastern black rhinoceros (Diceros bicornis michaeli)
18. Eastern bongo (Tragelaphus eurycerus isaaci (Syn.: Boocercus eurycerus isaaci))
19. European mink (Mustela lutreola)
20. Geoffrey’s spider monkey* (Ateles geoffroyi geoffroyi*)
21. Greater bamboo lemur (Broad-nosed gentle lemur) (Prolemur simus (Syn.: Hapalemur simus))
22. Javan Leopard (Panthera pardus melas)
23. Lac Alaotra bamboo lemur (Alaotran Gentle Lemur)* (Hapalemur alaotrensis (Syn.: Hapalemur griseus alaotrensis)*)
24. Malayan tiger (Jackson's tiger) (Panthera tigris jacksoni [= Nomen nudum!] (Syn.: Panthera tigris malayensis))
25. Mexican spider monkey* (Ateles geoffroyi vellerosus*)
26. Mongoose lemur (Eulemur mongoz)
27. Negros warty pig (Visayan warty pig) (Sus cebifrons negrinus)
28. Northern White-cheeked Gibbon (Nomascus leucogenys (Syn.: Hylobates concolor leucogenys))
29. Nubian red-necked gazelle (Nanger dama ruficollis (Syn.: Gazella dama ruficollis))
30. Red ruffed lemur (Varecia rubra (Syn.: Varecia variegata rubra))
31. Somali wild ass (Equus africanus somaliensis (Syn.: Equus asinus somalicus))
32. South-central black rhinoceros (Diceros bicornis minor (Syn.: Diceros bicornis keitloa))
33. Sumatran elephant* (Elephas maximus sumatranus*)
34. Sumatran orangutan (Pongo abelii (Syn.: Pongo pygmaeus abelii))
35. Sumatran tiger (Panthera tigris sumatrae (Syn.: Panthera sumatrae))
36. Western lowland gorilla (Western Gorilla) (Gorilla gorilla gorilla (Syn.: Gorilla gorilla))
37. Western saiga (Russian saiga)* (Saiga tatarica tatarica (Syn.: Saiga tatarica)*)
38. Western woylie (Western Brush-tailed bettong) (Bettongia penicillata ogilbyi)
39. White-belted black-and-white ruffed lemur (Varecia variegata subcincta)
40. White-collared brown lemur (Gray-headed brown lemur)* (Eulemur cinereiceps (Syn.: Eulemur albocollaris) (Syn.: Eulemur fulvus albocollaris)*)
41. Yellow-breasted capuchin (Buffy-headed tufted capuchin) (Weeping capuchin) (Sapajus xanthosternos (Syn.: Cebus xanthosternos) (Syn.: Cebus apella xanthosternos))

A very informative (though slightly out of date, the chinchilla's been downlisted) list. Many thanks @littleRedPanda !

@Swampy I compiled my lists in the summer and made a few errors at the time, as well as not double checking with IUCN