If you have UFC Fight Pass, you are eligible to watch UFC 252 Preliminary Fights. To open the access to the main event, you will need to buy the PPV from the UFC.tv. All you need to do is just hitting the order button in the UFC.tv. Choose the UFC 252 PPV package. Keep in mind that you could also choose to watch the bout a month later when it is added to your Fight Pass library.
Unfortunately, you won’t be able to watch UFC 252 in the BT Sport. Perhaps you haven’t known this yet; the BT spokesperson confirmed that BT company would end the relationship with the UFC on December 3rd. That means BT Sports will no longer broadcast the UFC events on the channel. The UFC 252 event happens on Saturday, 15 August, 2020. But you can still purchase the PPV through the service.