It's undoubtedly one of Britain's leading zoos (cos it's in London, innit) and it says here there are 15,104 animals. That's pretty impressive for a few sheds by the canal on the Camden borders.
The Zoo is slowly undertaking a renovation project to make the cages more like animals' natural environments, and this has seen the opening of renewed areas like the Blackburn Pavilion, which recreates a tropical rainforest atmosphere in what was the Bird House (the birds are, naturally, still there), Gorilla Kingdom and various other natural things.
It all sounds very nice, but it is still just a Zoo, and it's a bit steep at £13.90. Nevertheless, I do not doubt that it's a reasonable day's entertainment. The nearest tube is Camden Town and from there it's a pleasant walk down the canal, or along a busy road. You choose.
Here's the website: http://www.zsl.org/