Behaving British refers to the act of doing something you know you'll regret immediately after.
This term is a mock on Britons who voted for leave and took to the internet immediately after the referendum result was out, asking for a second referendum.
Many Britons who voted "leave" in the refendum expressed their regrets on social networking sites. Some said they didn’t think their votes would matter too much because they thought they were just going to remain.
Stop behaving British！ You've made the same mistake three times！
He decided to quit his job in the morning and regreted it in the afternoon. He is trully behaving British.