Copied from the Steam guide by rjdimo and Solara570.
- Start in China.
- Devolve any mutated traits for now on.
- Now wait patiently. There will be a pop-up for the Brexit event, which is entitled "UK votes for Brexit in referendum".
- Save game. See which kind of Brexit you want to trigger and refer to the corresponding section below, if you want to try the other achievements.
- Now, your disease should be spreading under the radar. Wait until UK is infected.
- Once UK is infected, evolve Insomnia, Paranoia, Seizures, Insanity, Cysts and Hyper Sensitivity immediately.
- Wait patiently. Before UK is fully infected, there should be a pop-up entitled "<plague name> triggers Brutal Brexit", where <plague name> is your disease's name. Meanwhile, "Brutal Brexit" achievement will be unlocked.
Note: If UK is fully infected, Brexit is likely to be cancelled. You'll miss this achievement, but you still have chance to unlock "No Brexit".