Given there is certainly no shortage of would-be zoo entrepreneurs in the UK, I suspect there are no more and no less zoos in that country than the market will support. In economic terms, the correct answer to your question would be the last one, because the number of zoos will always be approximately one less than the number that is unsustainable at that particular point in time. That's how a market works.
What I gather, though, is that you're concerned about market share. In the hope that you might be able to answer your own question, let me express it another way. Imagine you are a regular overseas traveller. Would you prefer that there were 5 large airlines operating out of Heathrow, each of which has roughly 20% of the market for flights in and out of London? Each of these airlines obviously has high revenue and a large fleet of aircraft to be able to fly so many passengers around the world, so each one offers a large number of destinations and an option between economy, business and first-class travel. The catch is, each of the five airlines goes to more or less all of the same places, and because they don't have a huge amount of competition, your fares are pretty high.
Your alternative scenario is, let's say, 25 small airlines servicing Heathrow. Each one only has 4% of the market on average, so they don't have a lot of income and they cannot maintain large fleets. Each one tends to specialise. Two or three might mostly do flights to Scandinavia and Eastern Europe, another couple might concentrate on the Middle East, for example. And because they probably have smaller planes, you only get the option of travelling economy. There is no room for business or first-class options.
Perhaps you can see how this correlates to zoos. In the first example, a few big zoos - lets say Chester, Edinburgh, Colchester, London and Twycross, for argument's sake - are dominant and get the overwhelming proportion of zoo visitors to visit them. Each one has a large collection of animals and a range of different extra services, such as restaurants, special animal experiences, souvenir shops, zoo rides and so on. The catch is - by trying to be all things to all people, each of these zoos turns out to be quite similar. There are less spaces for animals overall, for example, which means that fewer species can be viably maintained in captivity. And if one of the big zoos has the latest popular species, the others better get them too or the big 5 might become a big 4.
The 'small airline' example would be where you have lots of little zoos. None of them have the resources to be a dominant force and grab any more than their small local market as zoo-goers. As a result, what you see is on the one hand, less of the extra bells and whistles like zoo trains and multi-million dollar exhibits (these zoos only fly economy) but on the other hand, each one tends to do different things. So some zoos, like Dartmoor, become big cat specialists. Others, like Highland Wildlife Park, specialise in cold climate animals. Neither of these places are complete collections, but they do make for a more diverse zoo landscape.
In reality, Britain is a bit of a hybrid of the two (as is Heathrow, for that matter, with some airlines offering lots of different flights, like British Airways, and others maybe only doing one route). You have some big, full-service zoos. And you have a lot of little zoos that do a few things on a limited budget, but get by and add some diversity to the zoo landscape.
In Australia, the reality is much more skewed towards the the big airlines example. We have no more than six zoos in this country (excluding aquariums and theme parks) that can reasonably aim to attract 300,000 visitors in a year at present, and there are maybe four or five more that can attract more than 100,00. What that means, in part, is that our zoo collections are much, much less diverse than what you have in the United Kingdom.
I hope all of that made sense and you can resolve your own view on the matter. I know what I'd prefer.
