Obviously no zoo is perfect, and we should always be striving to improve animal welfare and standards with zoos, but they serve an important role in conservation, rehabilitation, research, and education. Visitors should always research a zoo before visiting to ensure it meets their own ethical standards, but most of the major UK zoos are, to my knowledge, pretty solid on that front. Durrell Zoo in Jersey is a personal favourite, alongside Marwell Zoo in Hampshire for their conservation work with lesser-known species such as the Przewalski's horse.
I appreciate that there's a critique on zoos functioning as entertainment establishments, but for many this is simply the best way to fund their conservation work while encouraging and maintaining public interest in species conservation. On a basic human level, people are far more likely to take an active interest in the welfare and protection of a species if they can see it and connect with it somehow. There is also the element of education; people may not be aware of why some species are endangered and how to support their conservation, and zoo literature often has a strong focus on informing visitors of these issues.
I think it's absolutely understanding why anyone, on a personal level, would choose not to visit or fund a zoo based on their own ethics, but to abolish zoos would be devastating to conservation efforts and would likely contribute to the extinction of multiple species. The best thing we can do is raise standards, improve welfare and prioritise conservation.