A local zoo to which the Dursleys took Dudley on his eleventh birthday, along with Piers Polkiss and, against their will, Harry (PS2). At the zoo, they bought ice cream treats, saw a gorilla, ate at the zoo restaurant, and visited the reptile house (which Dudley found boring because the animals weren’t moving around much).
PS/f incorrectly shows Harry and the Dursleys at the London Zoo. This cannot be true, however, because when Harry travels to Diagon Alley for the first time, it is stated that he had never been to London before (PS5).
Which zoo was it? There is a zoo/adventure park in Surrey that fits the bill. It's called Chessington's World of Adventure, and it has both a reptile house and a family of gorillas.