No need to go landside.
If you are picking up a BP in Italy, they will do the passport check then. If you are using a mobile BP or print your own BP, it gets checked as you transit airside at T5. You have to go through BP conformance check before you can reach the escalators upstairs to security.
If you do decide to go landside for a smoke or just to avoid the transit queues, you will have to go via a passport check desk. Landside BP conformance check wont do passport checks.