You're looking for species that aren't often seen for sale in the UK. The Liophis may be the most likely to find, from posts on other forums I'm aware that there are breeding pairs in the UK, and as they are the sort of snakes that appeal to garter keepers there may be people on thamnophis.com that keep them. Nerodia seem to be rarely mentioned, and the species I remember seeing advertised was the diamond back water snake.

Part of keeping rarer species must be the kick you get when you actually find what you're looking for amongst the thousands of pythons, boas and corn snakes that dominate the hobby in the UK. Happy hunting!