You have 2 options. Within the last month of your contract ring 789 - select thinking of leaving and give them 30 days notice
However, easier and in my opinion safer ask for a PAC code and give that to your new supplier. When your phone number is transferred your Virgin account will end. PAC codes must be used within 30 days.
If you do nothing you contract becomes a rolling 30 day contract and you will continue until such time as you terminate