FWIW, a similar thing happened to me a couple of months ago. I was towing my caravan and wanted to take the second exit from a roundabout. The young lady driver in a Pugeot 206 waiting at the entrance between stopped to give way to the car in front of me and then pulled out to turn left as I was alongside her. I couldn't stop behind her and was already commited to the exit, so I pulled out as far to the right as I could. Both of us stopped with her car neatly trapped alongside my rear wheel, unable to go forward because of the nose of my car and unable to go backward because of my trailer. I had to reverse about ten feet before she could move!
I wasn't sure whether we'd collided, so I asked her to pull into the layby a couple of hundred yards up the road. There was no damage to either vehicle, so the I must assume the thunk I felt was the trailer brakes applying more ferociously than ever before or since! She claimed she hadn't seen me (a LWB 4x4 towing 6.5 metres of caravan!) Since I would have been head-on to her when she should have looked right, and since most people look where they're going when they start to move off, I could accept this was down to "killer pillars". Anyway - no harm done and I left her with the URL for
www.smidsy.org.uk