In the past couple of years i have visited the following zoo's. I have listed next to each where i found the photo op's to be, of course different days may provide different opportunities.
*Marwell - Giraffe, Amur Leopard, Lemurs, Meerkets, Macaque, Penguins, Colobus Monkeys.
Port Lympne - Gorilla's (check out the group of Silverbacks!)
Whipsnade - Cheetah, Lions, Wolves, Elephants, Lemurs, Penguins and the odd wild bird of prey catching air on the hills.
Woburn - Lemurs, Wallabies, Lions, Squirrel Monkeys, Giraffe, Rhino.
*Bristol Zoo - Gorilla's, Penguins, Inca Turns, Fur Seals, Prairie Dogs, Lemurs.
*Twycross - Gorilla's, Prairie Dogs. Note that there is a new Snow Leopard section and a new Elephant enclosure opened since i visited.
Cotwolds - Lemurs, Lions, Penguins, Meerkats. TP user Cosmix will be able to give you more info on this place!
*Colchester - Giraffe, Zebra, Elephants, Orangutans, Mandrills, Tigers, Wolves, Meerkats, Golden Lion Headed Tamarins.
WHF - Cheetah, Lion, Leopard, Tigers, Lynx, Serval.
Longleat - Otter, Lions, Giraffe, Wolves, Rhino.
* = admission free with Marwell annual pass. Colchester gets my vote as the best zoo i have been to. Alot of the enclosures are VERY photographer orientated so there are plenty of ops.
I have also visited Lisbon Zoo and Orana Park in New Zealand, but these are a little out of the way!!