I'm glad to see London Zoo offering a cheaper admission price, and I wish more zoos across the world would follow suit and either decrease their admission or offer more opportunities for free/reduced entry for those of need. This gets to the heart of the role of zoos in society, in my opinion. Zoos (and aquariums and museums, for that matter) should be, at their most basic level, community institutions, as many zoos were built with community support and funding, and a lot continue to receive support as well. As such, zoos should be engaged institutions at the community level, and a great resource for all members of the community, regardless of income. Especially keep in mind the fact that for many families admission prices add up quickly. If a zoo ticket costs $20 for one person, which seems like a rather standard price, then for two parents to bring their three kids it'd cost $100 for admission to the zoo. Sure, that's not as expensive as a trip to the amusement park, but it's still outside of what many families can afford to spend on a recreational activity.
There are also examples of zoos that are free and still thriving, such as Saint Louis, Lincoln Park, and Como Park Zoos in the United States (plus National Zoo, but they're a different story since they get federal funding). While I don't know the breakdown of revenue for these zoos, it is clear they are valued members of the community and still manage to exist, and build new exhibits, despite the lack of an entry fee. Zoos provide an invaluable service to the community, and it's a shame to see some institutions take on a sort of elitism where only those who can afford are welcome. In an era where access to outdoor spaces is at an all-time low, and people are living increasingly indoor, sedentary lifestyles, zoos should be doing everything they can to welcome all people to access their outdoor spaces, walk around, appreciate nature, and hopefully learn something as well, not further limiting access to people who can afford the luxury of a day to the zoo.
Instead of making most revenue through admissions, zoos could (and should) shift some of that burden to other revenue sources. Charge for parking, for instance, or incorporate more paid attractions within the zoo. Alternatively, plenty of zoos have started to embrace special events and night access, to see lantern festivals, holiday lights, etc., and a further embracement of special events and other opportunities should allow zoos to match their current revenue earnings while decreasing ticket prices. Perhaps offering multiple levels of admission could also work, allowing visitors to visit the zoo's ground for free, but pay an extra fee for access to indoor exhibits, or certain marquee attractions, so that even those who can't afford to get the whole experience can at least get some zoo experience.