After you have booked your ceremony, you are both legally required to give a Notice of Marriage, in person, at a Register Office.
If you are residents of Islington or the City of London you can book an appointment to give notice on-line here.
If either if you are foreign nationals, subject to immigration control, you must give your notices at a designated register office.
Therefore you can also book an appointment to give notice on-line with Islington and City of London Register Office here.
If you are both EEA nationals you will need to contact the local register office where you live in order to give notice.
More information on giving notice can be found on GOV.UK