Whilst normally I would agree with the others in that if someone drives into the rear of you it is called strict liability and the rear ender would be liable. However.....
1st question. Why did you stop on the major carriageway when the pedestrian was on the opposite side of the road and nowhere near a pelican or pedestrian crossing?
Conditions were not very good, visibility was reduced and you stopped suddenly when there was no need to.
Whilst the other driver will have to accept the lions share of the causation, you might have to accept a degree of contributory negligence for the reasons I have mentioned.
Had it been for a pedestrian or pelican crossing, it would not be an issue, but to stop suddenly where you did for a pedestrian who was on the opposite side of the road and therefore did not require you to stop, it puts a different complexion on things.
So as mentioned, all you can do is let your insurers deal with it, but do not be surprised if they settle on split liability.