And again today - I came out of a side street, needed to be in right hand lane, so moved over into but was at bit of an angle with rear into left lane. So straightened up by moving closer to car in front... with still about a metre to spare (by which time I was only in the right lane by cms - still on an angle) the car in front suddenly lurched forward (into admittedly plenty of space - 3m or so) as if I was about to hit him and kept glancing back, seemingly worried, afterwards. 
If I was in that situation, I would also move forward. I prefer to keep as large a buffer zone around me as possible, and to assume that all other drivers will do something stupid or make a mistake.
As do I too, when practical, like a large buffer zone - for the same reason. Likewise I don't like having my tail sticking into another lane, even slightly, in case some dumb a** beetles a long and doesn't notice it some how.
My point was more the fact of the 'sudden lurch' and repeated glancing (head movement towards mirror) back - as if I almost hit him. He appeared to have spotted me when I first pulled into the lane - so it shouldn't have been a surprise, and why didn't he move forward into the large amount of space (he still had a good distance between him and the car in front when he stopped after the 'lurch') to allow me fully into the lane without me getting 'too close'.
In fact after the lurch, due to it and the repeated glancing, I didn't move any further (I had stopped just before he lurched, as by that point I was fully in the lane by cms) and left plenty of room when we took off after the lights changed - as I judged I had 'rattled' him. Which seemed to be confirmed by him continuing to glance often as before, even though there was now sufficient space for another car to safely get between us in that lane. Actually I felt a little bad/guilty that I had rattled the driver.
