About 20 years ago I drove into the back of someone who panicked when an ambulance she clearly hadn't seen drove past with the sirens and lights going. It was a dual carriagway with a 40 mph limit and I was in the same lane about two car lengths behind her car, with the ambulance going past on the outside lane. When she saw the ambulance she did an emergency stop and despite me braking hard I still hit her car. She was livid and and got out shouting (ironically) "did you not see that ambulance?". Having heard all the stories of rear ending someone always = your fault, I assumed I was going to be 100% liable.
It seems not, because the insurers deemed it was her who was liable. I can only assume her witness statement corroborated that we were both driving on the inside lane, with the ambulance overtaking on the outside lane and that she did an emergency stop with no actual hazzard present.
Sounds like you can add yourself to this thread for tailgating.