– Silverstone, UK

When you read “McLaren Senna Validation Prototype 736,” that middle bit doesn’t sound particularly sexy or exotic. “McLaren” conjures up decades of Formula 1 teams and mind-bending road cars; “Senna” calls up images of one of Earth’s greatest ever racing drivers, probably staring off in the middle distance somewhere, looking epic. But “Validation Prototype” sounds like an electric shaver or a new recipe for protein drinks, as much as it does a near priceless early version of a supercar that someone has foolishly agreed to let me drive around Silverstone… in a light rain.

There have been moments over the last decade-plus of my car-writing career when I’ve looked around and thought, “where the fuck am I?” And after getting first packed into a set of Richard Mille-sponsored racing overalls, then five-point-harnessed into this engineering prototype of a million-dollar, carbon-fiber, land-based rocket ship, that phrase is bouncing around my skull.

It’s one thing to drive someone else’s supercar at the track. This is quite different: McLaren built this car as the original demonstrator of the Senna  – “It was only supposed to go round the lake” at the McLaren Technology Centre, we are told – but has been continuously refining and improving it as the final spec for the production cars has been developed.

In fact, despite its very production-car appearance, inside and out, VP736 is very much an engineering work in progress. The final version, which I’m already i...