There are at least twenty pure zoos dotted around the UK so there is no excuse for not going to a zoo for a photoshoot. Like most fields of photography, planning and forethought can contribute greatly to the results that are achieved.
Look at the website for the zoo that you are planning to visit and make notes about opening and closing times.There will almost certainly be a number of talks and other events taking place during your visit so try to make some kind of itinery rather than just wandering around. Look out for talks that include feeding as most animals tend to be docile for most of the time but hunger and the prospect of food is a great motivator. Get to the talk a few minutes early to allow time to find a good viewpoint. Take some test shots thinking about the kind of images you are looking for. Choose the lens that will give you the best results. Other things to think about are colour balance, direction/quality of light and the background elements you want in the shot.