What would you like to do after the cancellation? Will you be keeping the phone number? Remember you can always upgrade to a new phone or a sim only deal through the website or the My O2 app!
If you don't wish to keep the account or the phone number, you are still able to cancel via customer services on 202. But please bare with them! They are extremely busy at the moment as i'm sure you can appreciate. It may take a while to get through but they are still there!
How to cancel here without calling Guide: Cancelling Your Contract