If you petition on the basis of your spouse's unreasonable behaviour, you don't need their active cooperation to get your divorce.  If they fail to tell the court that they have received the divorce papers (by completing, signing and returning to the court the 'acknowledgement of service' form that they have received with the petition), you can arrange for the papers to be served personally.  The process server hands the papers to your spouse then swears an affidavit giving the details of when and where service took place and explaining how he knew he was serving the right person.  When you come to apply for decree nisi, you rely on the 'affidavit of service' instead of producing a copy of the acknowledgement of service form and identifying your spouse's signature on the form.