As first thing you will need to communicate your intention to get married, at your local registry office in the UK, they will issue you then with a certificate of no impediment.

Once you have these, you must send the originals to the competent consulate, by registered mail together with the documents listed above.

Please visit the Foreign Offices page UK in Italy – page – MARRIAGE IN ITALY, where you can find several scenarios in which your case may belong and consequently, you will find further information:

http://ukinitaly.fco.gov.uk/en/help-for-british-nationals/living-in-italy/marriage-guide/