You mean like the dead-of-winter every Monday to Friday in
June?
I should have said usually, but if you wish to be pedantic, sure, you could find £47 tickets throughout the year. The underlying issue of things being closed on these days remains. Parc Disneyland's premiere attraction, Phantom Manor, remains closed until late October. To insist on full price until it's reopened is nuts as there isn't enough to do when three or four other attractions are also routinely closed.
I may be being thick here and not understanding the point you are trying to make - I'm lost as to whether you're arguing that Disney is better or worse value than London Zoo - but the argument that visitors have to buy food and souvenirs seems a ropey one to me. Such secondary purchases are all very nice if they can be afforded, but they're by no means an essential part of a zoo visit. Certainly, when I am feeling skint, I won't be purchasing unnecessary plastic objects of dubious taste in the zoo shop, nor over-priced food in the cafe.
If I may, I believe you're arguing from a localist perspective; i.e., somebody who can and does, in this example, visit ZSL - London on a regular basis. I'm approaching it from a tourist's perspective for whom a trip to, in this example, ZSL - London is a once in a decade chance.
In this instance, the tourist is likely to spend the entire day at the zoo. As such, they're likely a captive audience; food is going to be required irrespective of cost especially if children are involved. Souvenirs are likely as well. Dubious plastic tchotchkes or otherwise. I'm not going to fault the visitor on their choice of consumption at this point in time.
I personally find ZSL - London to have plenty of value for it's cost, but for the typical visitor from North America, Disney and Disneyland are a known commodity. It won't be the exact same thing, but you know you're going to have a high quality experience. ZSL - London? They have lions. The typical North American tourist can see those at home for infinitely less at their home zoo. It's the specialized traveler, the one that posts here on Zoochat, that cares that ZSL- London's lions are Asiatic with a new whizbang exhibit.
Perceived value relative to cost is important. To pick, perhaps, a closer example: If the Wizarding World of Harry Potter gets built near or in London in the next couple of years as rumored, and has a comparable cost, which is going to be perceived as having more value? One has lions, the other is what England is "supposed" to be. Y'know, "quintessentially English" things.
That Julian Barnes novel
England, England comes to mind.
People often complain about the cost of following football by referring to the price of food, drink, replica shirts and so on. I feel the same way about this line of argument - just because I choose to go to see a match, I don't have to buy a pie, a drink, a nasty nylon shirt.....
I both understand and share this line of thought. I've largely given up on organized sports, but for my local zoos I rarely buy souvenirs or food. I'm also not going to spend the full day each and every time out. Popping in for a couple of hours for something specific is typical.