You can't. I had the same problem, box was live according to Openreach but not offered by BT (or anyone else). It was nearly 9 weeks before BT offered the product. Apparently this is not unusual, you just have to be patient, my service appeared on BT's website on the first Monday in Sept. There seems to be a long delay between Openreach updating their site & the ISPs getting the updated information.
As I mentioned above, it was several weeks before BT offered Ultrafast once the cabinet went live. 2 months on & I've just learnt via a neighbour that TalkTalk ultrafast still isn't available in our area, so he's looking to move ISP. So it looks like the ISPs are dragging their heels when it comes to G.FAST.