I personally view that a last birth you cite for 2017 is not exactly a regular breeding occurence. I would not mind a more statistical listing of common hippo in the UK and where its population development and breeding the species is concerned.
I can't provide a statistical analysis like you request, but I can say Hodor's birth at Whipsnade was certainly
not a statistical anomaly.
Whipsnade have a history of successful breeding that continues into the modern era, but are hampered in numbers by the fact for most of the past 20 years they have held a female pair and a geriatric. Until Lola's mother died (I believe her name was Nigna? Although I may be skipping a generation), there was insufficient space for a calf. It's telling that once she was euthanised, the breeding pair successfully bred. Hodor will remain with his mother for a few more years which prevents her from breeding again, hence why Hoover was moved. And within a few years of his move he's already fathered another calf.
West Midlands I believe have a non-breeding group, and from what I've heard it's not exactly a brilliant exhibit for a breeding group. Longleat has lost both their females, both of which have successfully bred when younger, and intend to replace them with a breeding pod. Flamingo Land needed a breeding bull, which they now have.
The issue with hippo breeding is not one of inadequate concern but more of a lack of male hippos and holders in general in the UK. If Whipsnade had managed to obtain a male earlier I'd imagine that they would have had a calf earlier, but you can't really say that for sure. They're large mammals, they require lots of space, have long gestations and require calves to stay with their mothers for years. I know for certain Whipsnade intends to breed again once Hodor is moved, and ultimately he will provide another bull to collections (even if it keeps the gene pool relatively narrow).
TL;DR: Common hippos breed well in our zoos and their holders intend to continue doing so, but there have been smorgasbord of issues that has slowed hippo production.