No point hooting at someone who has already hasn't seen you and already pulled out (your horn will not make their vehicle disappear) , as they are already in the path of your vehicle, if you can carry on at same speed and not hit them there is a macho victory of sorts to be had by hooting at them.
If I'm coming towards a junction, and I see someone on a side road start to move, or I see a parked car start to pull out, I'm going to hit the hooter and cover the brake, if it's far enough away. You see, I can actually do these two things at once, but only if I have the time to see it coming - which is exactly what I said earlier...
I try to get to the hooter if I can see it coming in time, it sometimes stops them in their tracks. Then again, I guess it depends how loud their radio is.
Of course, in the instances when a vehicle is suddenly in your path, the hooter is the last thing on your mind, and some sort of avoidance is in order. This is where instinct kicks in and you react. I'm not going to fumble about for the horn whilst ploughing into them, nor did I ever suggest that.
But the first thing is BRAKE. In fact in a parody of 'mirror-signal-manoeuvre' there is 'brake-swerve-horn' .
The first thing you have to do is trust your instincts. There are times when you brake, times to steer, and there are even odd times that I have put the foot down to avoid other road users. As Jocko said earlier, you can't really say what you should do in a situation like this, it's just a reaction based on your instincts of self preservation.
I know you mentioned tailgating earlier, I had one yesterday on the A614 Goole to Driffield road. Some complete arse in a black Audi, a few feet off my rear bumper for several miles and couldn't get past. He was obviously not paying a lot of attention to what I was doing as he didn't notice me indicating left, in plenty of time might I add, and started flashing his lights furiously at me when I braked for the turn.
Trouble is, with someone like that behind you, even if you do brake to avoid a collision, they'll hit you before they get their foot to the brake pedal and push you into the obstacle anyway.