I am growing more and more uncomfortable with zoos.
Lots have a 'conservation' drum to bang... or a rescue one, or both - but when you dig deeper, you find that they are conserving animals to... sell/share with other zoos.
These are the species with zero chance of return to the wild and no effort made to do so, because there is no wild to return them to - or species that do not need wild numbers boosting in the first place.
Some are pretty prolific breeding programs... because they can breed 'em but they can't actually keep them alive all that long so theres a constant need for replacements.
Lots of animals are culled, when they don't fit a breeding program anywhere and zoos are out of room as a result of the need to keep breeding - giraffes spring to mind.
Some species live very well in captivity - sedentary species who lurk waiting for food to pass by, snakes for example, seahorses do quite well, herd dwelling herbivores are pretty safe stuff.
Some breeding programs and research programs are valuable - these tend to be small, boring, icky, un-pretty creatures that few will ever notice though.
Lots of places run under the guise of rescue, a dolphin facility I can think of as an example... has rescued (last I heard and had to sit through a 3 hour sales pitch lecture) 3 from the wild, in the entire time they've been running (decades).
All their other 'rescue' dolphins were 'rescued' by handing cash to a failing/closing down facility elsewhere. They do no actual research, they make big bucks flogging 'stay three weeks and learn to train dolphins'... to boost your CV or ego. They have a vague wafty hint at 'return to wild' schemes but to date have not actually done this, theres always a reason they can't...
They of course make a big deal about using human training methods - but this should be the base line for acceptable methods, not a special thing to trumpet about!
So the bottom line is most zoos in the UK (and they are by far the better ones on the whole!) are conserving animals for the benefit of the public and other zoos.