First off, McLaren does note that this will be the most aerodynamic road-going model that they have ever created. To give us an idea of how the aerodynamic will look, they have released a teaser image of the car. This image can be seen above.
McLaren Special Operations (MSO) will be creating BP23, with development already underway. Even though the car hasn't even started being produced, all 106 examples of the car have already been pre-sold. Why 106? This is the same number of McLaren F1s that were created back in the day.
Expect the first McLaren BP23 examples to be delivered in 2019.