The demise of the New Forest tree frogs from in and around a pond near Beaulieu in the late 80's after at least 40 years, was probably caused by over-collecting, a large population of GCNs, and an invasive alien plant (New Zealand stonecrop), which chokes the pond during the summer. There are no "official" documented sites for Hyla in Britain now, but I have heard of 2 colonies of arborea in the south of England, and possibly one near Newbury, Berks of H. savignyi.
Killian- presumably they wouldn't have protection from English law, as they too are considered alien species.
Ed- my small collection of animals is kept within a semi-rural hamlet, my garden being just 1/3rd of an acre (unlike Philadelphia Zoo).
A greenhouse full of tree frogs making a din all night, every night, which is best described as a mix of ducks quacking, and dogs barking, is quite disturbing to my immediate neighbours, (unfortunately). They are perfectly happy with calls from parrots and macaws during daylight hours, and eagle owls through the night, but not the tree frogs.
I know of a population of Midwife toads (Alytes obstetricans) near High Wycombe, Bucks, which has been established for 20 years now.