Robin Hill country park had saltwater crocodiles for a time, until the mid-nineties, and it closed as a zoo soon after that. I've no idea where these animals went, they were not fully grown when I saw them.
Thrigby does indeed have American Alligators. A similar house to the current swamp house there is being built at it's sister collection, Cromer Zoo, and when this collection opens I expect the alligators will move there. Noah's Ark Zoo farm may also have just recently aquired some juvenile american alligators, I'm not 100% sure.
The American Alligator at Beaver Water World died I think last year or 2005. For years this animal enjoyed possibly the best alligator enclosure in the uk, a large, heavily planted outdoor pond in summer, with plenty of land surrounding it. They built a concrete outdoor pool as an attachment to its indoor winter exhibit, presumably due to the stress of moving the animal twice a year, but it died very soon after this was constructed. Apart from the old alligator pond, which I suspect was accidently a brilliant enclosure as it had been left to become so overgrown, Beaver Water World is really the nearest thing we have to a 'Roadside Zoo' in this country, and is a pretty shocking place.
What happened to the False Gharials about 10 years back at paignton? They were huge, really amazing creatures. Did they die or were they sent elsewhere?