Then you're just not thinking it through. I don't know the details of this incident but...person falls off train at Newhaven. Guard attends to person who has broken his leg. Guard cannot leave this person until at least emergency services arrive and possibly beyond as a duty of care to passengers. Train cannot go anywhere without the guard on this line. Train behind blocks one line at Lewes and train behind that cannot get through Lewes. Guard and driver of the first affected train at Newhaven may have then been scheduled to work the next eastbound train from Brighton but are still stuck at Newhaven so that train is cancelled OR may legally be required to take their break when they finally get back to Brighton so they can't work their next train. So, within a few minutes on a very crowded network at a very busy time you have trains and staff at the wrong place at the wrong time and chaos descends very quickly.
Then why design a rail system which is so prone to the slightest issue? I'm sorry but there's no excuse, they've had 180 years to get this right after all.