One P1 driver experienced that sort of frustration while running Silverstone, when he tangled with a very persistent BAC Mono driver. Now, the P1 absolutely decimates the single-seat, closed-wheel track toy on the straights, while losing a lot of ground in the bends. Even on a high-speed track like Silverstone, the McLaren doesn't boast enough of an advantage to keep the Mono at bay.
As the intelligent YouTube commenters (yes, we're surprised by this too) point out, though, there could be a number of reasons why the Mono is so able to hassle and pass the P1. First, it's quite clear that the BAC driver knows the circuit far better than the McLaren pilot, as evidenced by his lines, particularly through Stowe and the Club/Vale complex (this could be chalked up, as one commenter says, to the lead car driving with his mirrors). The BAC is probably running a more aggressive tire than the P1, which in turn appears to be handicapping itself by not running its most aggressive aero setup. Finally, remember that the McLaren is a fully street-legal vehicle, full of safety gear and crash-worthiness as required by law.
Still, the fact that this relatively affordable track toy is able to hound and pass one of the world's premier performance machines is a particularly impressive feat. Take a look at the video to see what we mean.