Basta sair antes de expirar.
Embaixada da Itália em Londres diz o seguinte:
1.2 UK Passport validity for travelling to Italy Foreigners may travel to Italy provided their UK passport is valid when they exit Italy (no minimum validity required). This applies to all EU passport-holders. If you have a non EU passport, please consult the website of the Italian Foreign Ministry to verify whether you need a visa to enter Italy