Following up on some points already made I too find contemporary British collections a bit disappointing.
"When I was a lad" the 'big' collections [Regents Park, Chester, Belle Vue, Bristol] offered a huge range of species. It's become known, disparagingly, as a "stamp collection mentality" but it did offer the visitor an opportunity to see a wonderful variety of wonderful creatures. The housing was often poor, but there was plenty to see!
Today the larger collections now offer a greatly slimmed-down offering, and the smaller collections offer much the same. But less.
Very accurate appraisal here of what's happened to many of our Zoos. Most people would say they've changed for the better as more spacious housing and better animal care have replaced the old- but in nearly every case at considerable loss of collection diversity, as you say.
I remember Belle Vue Zoo- a fantastic collection. London, where in those days you couldn't expect to see everything properly in one visit as you can nowadays. Bristol, where you could see virtually every ABC animal species including Rhino, Elephants, Giraffe, Zebra, Antelope, 3 Great Ape species, about a dozen of Monkeys, a whole range of large cats etc that's without including the Birds, Fish and Reptile collections.
And I am sure those collections will never return.