If your passport is lost or stolen you may complete the relevant form (pdf/doc) and send it by post together with a copy of your ID document to the following address:
Ufficio Passaporti
Denuncia Smarrimento/Furto Passaporto
Consolato Generale d’Italia in Londra
Harp House, 83-86 Farringdon Street
London EC4A 4BL
The above form is also available at the Consulate and can be submitted together with your passport application.