You will either have to risk it getting seized or try to import for your pets. There is a poster here who can help you. The 2016 Psychoactive Substances Act unfortunately blanket bans almost all psychoactive substances.
Make sure to write to MP and/or the relevant minister, explain the truth about kava, and make clear how ridiculous the regulations are. If people don't make a fuss about this stuff, there's not a hope of getting it changed.