i agree some trops / temperate fish sold as trops could probably survive in the very south of the country ,esp with the mild winters we are having and considering more and more tropical saltwater fish are turning up on the shores of the south coast , but i dont think they would breed though. although some species have a way of proving us all wrong , like already said their is already breeding populations of red ear terrapins around the UK , there are many breeding colonies of ring neck parrots living in the south ,and there was even a group of dwarf caiman found living in marshland around sunderland , there is a couple of groups of wild bore confirmed (where the hell did these come from) ,the rivers already have loads of non-native carp inc. koi and goldfish ) so i wouldn't rule anything out