That’s a really difficult one for the insurance company. The problem you have these days is that car wheels generally don’t lock because of ABS, and as a consequence the tyres won’t leave traditional locked wheel marks. They will leave ABS scuff marks but they will not be obvious to the untrained eye.
That said, sight lines, as you rightly say, should give them an enormous clue as to the other vehicle’s approaching speed before any braking. Put simply, as a driver emerging onto a main road you cannot be expected to react to that which is not there to be seen. On a most basic level, you would expect that any vehicle which is out of view when you emerge from a junction which then collides with you really has to be exceeding the speed limit at some point, because those speed limits are set giving consideration to these very issues - if there is a tricky junction where drivers are unsighted there is generally a reduction in speed limit, or at the very least a warning of some sort to drivers approaching on the main road.
It sounds very much as though the insurance company in this case, in the absence of any independent witnesses, have simply applied the balance of probability and decided the most likely cause was inattention on your part, as the driver emerging from the minor road. Or, in layman’s terms, it was in the too difficult to deal with tray.
Glad you were unhurt, which is by some distance the main thing. A car is a lump of metal.
9 trifles ordered and paid for and 8 turn up. 40 cups for tea and coffee for 90 folks. No teapots out and the two I eventually found were close to growing mushrooms in them. 
