I used to visit Tilgate several times a year and it is a surprisingly good little collection. It used to be free entry. It also used to be a rare domestic breeds centre. I know its bird collection has improved greatly in more recent times, breeding many fairly tricky species in fact. It is a little gem and very underrated even locally. Yes it is a small collection but it makes for a nice relaxing afternoons visit in good weather!
I guess the charge is a sign of the times, with Council budget cuts everywhere, but I don't think many would begrudge paying the very small entrance fee, in fact with the introduction of a few more (popular & easily obtained) mammal species, at a small investment, they could easily double the charge without putting people off visiting.
As Nanook has said, it is predominantly a bird collection, with over 100 species. I can't remember all of them, but there were around a dozen pheasant species, over 20 waterfowl, Stanley Crane, burrowing owl,raven, 2 laughing thrush, kookaburra,bali starling, java sparrow and various lorikeets and conures plus plenty more. The best aviary was a large glass fronted one for Waldrapp Ibis, White Naped Crane and Little Egret.
The mammal collection was somewhat limited:
Harvest Mice, Degu, Azaras Agouti, Red Squirrel,Parma Wallaby, Muntjac, Red Deer,a large paddock for Mara & Alpaca and various domestics - Cattle, sheep, goats , pigs, rabbits guinea pigs. The new enclosures for meerkats and Asian Small Clawed Otters were pretty good for a collection of this size.
There were a few invertebrates, amphibians -3 species of dart frog, whites tree frog, axolotl and ribbed newts. The reptiles on show were Dumerils & Jamaican Boa, Utila Iguana, a couple of small tortoise species, spiny tailed lizards and European Terrapin.
Nothing particularly rare, but a pleasant (& cheap) 2 hours even on a damp autumn afternoon.