Depends on your attitude to risk and how comfortable you'd be to risk losing the discount option by taking it all the way.
You did stop with the rear wheels in the box. But the angle of the camera makes it very difficult to judge if you had space to move forward the additional couple of feet necessary to clear it completely. If you did, then no contravention occurred when you entered the box junction.
Further, the video starts shaking and zooming just as you are coming to a halt so we also cannot easily confirm that the car in front of you was just starting to pull away. It's only a contravention if you stop in the box behind a stationary vehicle.