London zoo is best compared to other London tourist attractions, rather than other UK zoos. London is going through a period of extreme, rapid gentrification, and its tourist attractions are very much a day out for moneyed visitors or middle/upper-class Londoners. Central London is on its way to becoming a museum of itself, and based on observations during visits, I think low-income families are one of the demographics least likely to make a trip to London zoo, especially when Paradise Park and Colchester are easy to reach, or if you're going to blow a huge sum of money then Chessington would provide a theme park as well. People on low-incomes are being priced out of the central boroughs, and soon it will be easier for many to reach Paradise Park or Colchester in terms of transport links than it is to get to Regent's Park. Perhaps in recognition of this, the previous Mayor, a well-known champion of ZSL, did have a subsidised scheme going to ensure every London schoolkid got a free trip to the zoo, but this was scrapped by the current Mayor. But hey, everyone loves Boris, right?
There are many London attractions where there is no 'free' public space to eat food you've brought with you (i.e seating only in cafes etc), with the zoo this isn't the case and you can find somewhere interesting to sit and eat while watching animals. As kids we never bought food inside the zoo.
Also, unlike many top-tier London attractions, ZSL actually pay their catering staff above the living wage, kudos to them for doing so.