Mostly a theoretical question because the accident didn't happen (just!).
Country A-road, 60 mph limit. The driver in front of me has been doing 40 mph for 3 miles, with no safe place to overtake. Finally a safe overtaking place opens up, I indicate and start to pull out.
The car behind me has had the same idea, and plans to overtake both of us. He doesn't indicate and pulls out. I swerve back in just in time. No collision happens.
However - if I had pulled out at the same time as him and we had collided (he would have hit the back of me, but also probably the side), who would have been at fault?