For one thing, we have "hybrid" and "hybrid prototype" stickers, which can be notice if we zoom in on the test car.
However, if we focus on the windshield decal, we'll notice this reads "MVY04-BP23", which just happens to be the codename of the McLaren Speedtail (you know, the second McLaren hybrid, which followed the gas-electric P1).
So it looks like the Woking automotive producer could be testing the Speedtail powertrain in the body of a 720S. As such, it's reasonable to expect this prototype to be a tester for the 720S successor.
Brands like McLaren and Ferrari have already accustomed us to a new model strategy that involves a solid evolution with serious links to the replaced car rather than all-new models, with the 720S (we can trace its tech bits back to the early MP4-12C) and even the SF90 Stradale (the same is true here, with the 458 being at the top of its "family tree").
So the replacement for the McLaren 720S could use a derivative of the McLaren Speedtail powerplant, which mixes a 746 hp 4.0-liter twin-turbo V8 with an electric motor generating 308 hp, for a total output of 1.035 hp.
While this places the Speedtail in the same output league as the Ferrari SP90 Stradale, the first is a three-seater Grand Tourer, while the latter is a supercar. So all McLaren needs to do is use a similar powertrain in a package optimized for performance instead of continent blitzing.
And with the Britsish carmaker's Track25 business plan involving 18 new models by 2025 (all Sports and Super Series model will be hybrids by then), it looks like we'll get to enjoy plenty of flavors.