Star a game on Normal difficulty with Bacteria.
First step
1. Start in China.
2. Devolve any mutated traits that happen.
3. You now have to wait for a pop-up for the Brexit event, which is entitled "UK votes for Brexit in referendum". Now its important that the UK does not get infected before this event. If the UK does get infected by more then 1%, then start again.
4. Save game here. You can use this save to go for the other Brexit achievements.
Second Step
1. Now, your disease should be spreading. Wait until UK is infected.
2.Once UK is infected, evolve Insomnia, Paranoia, Cysts, Hyper Sensitivity and Cold Resistance 1.
3. Before the UK is fully infected, you have to wait for a pop-up entitled "XXX triggers Hard Brexit". Achievement will then unlocked.
If the UK gets fully infected before the "XXX triggers Hard Brexit" pop up. Brexit will likely be cancelled. You will fail this achievement, but you should get the "No Brexit" achievement instead.