The BP23 Prototype is actually a 720S that has been heavily modified, with the biggest modification being the seating arrangement. Back when the Hyper-GT was announced, McLaren noted that it would be a true F1 successor with the iconic 1+2 seating. Inside the BP23 Prototype, McLaren has reintroduced the single-seater arrangement.
In the images of the BP23, we see other aspects of the interior that are interesting. In front of the driver we see a myriad of nondescript switches, as well at the folding driver display that was introduced in the 720S. Also, there are digital sideview mirrors being tested which are being considered for the car.
This is all the information we have about the next-generation McLaren Ultimate Series car, but we'll be keeping everyone up-to-date with information as it's released.