In order to obtain a Divorce in England or Wales, you and your spouse must a) have been married for at least one year b) have grounds for divorce, c) meet the residence rules; and d) have a marriage recognised as valid by UK law.
When you apply to the court for a divorce, one of the documents that you need to supply is your marriage certificate. If you have lost it you will need to apply to the General Register Office for a certified copy.
Many of us are in a state of shock when we start the divorce process. You don't really take it all in at first. Its not until much later that you can look back and start to put it all in perspective, and draw out some key learnings.