DASHCAM - On the approach to the 40MPH section, of the M27 approaching the Hamble River, a motorcyclist (Registration HX74 VTJ) aggressively tailgated me. He as literally within five or six feet of my rear. As a motorcyclist myself, I knew I couldn't brake as to do so would have almost certainly have resulted him crashing into the back of my car, and I couldn't move left because the lane to my left was already occupied by a black car. I came off the accelerator to naturally slow down and then the black car to my left sped up. I was about to pull over to the left to allow the motorcyclist to pass when the motorcyclist aggressively undertook me and pulled in front of me, in a VERY narrow gap. I flashed at him and he gestured angrily at me as he sped away (doing way more than 40MPH).