I agree, quality over quantity always, and I think smaller more conservation focused collections are much better than big ABC theme park type zoos.
I don't know a lot of UK zoos but I would say that Jersey Zoo (simply the best in my opinion, in all regards), Shaldon Zoo (small animals, intimate and a lot of heart and commitment to supporting conservation) and Bristol Zoo (commitment to smaller and endangered species) stand out to me as being among the best.
ZSL is impressive to me in terms of the history and some of the smaller and lesser known species they keep and involvement in in-situ conservation but I wasn't that impressed at all with the direction they are taking at the zoo, the garish aesthetics of the site, and seemingly nebulous leadership.