I'm in Leeds and small goldfish that have been bred outdoors survive in small ponds here. I'm not sure how you would successfully acclimatise indoor fish to a life outside other than gradually. People who keep them would advise.
I believe that the Asian Weatherfish Misgurnus anguillicaudatus can legally be kept in aquariums in the UK but a permit is required to keep them in ponds. They were found to be reproducing in a pond in Southern England and the population was subsequently eradicated by drainage and liming. This is thought to be an isolated case, but it was put on the restricted list after this occurrence.
The European Weatherfish Misgurnus fossilis has been found 3 times in the wild in the UK but is not known to have reproduced. It is unlikely to establish now that it is no longer imported.