We saw the aftermath of a couple of accidents. Just cuts, grazes and bruises I would think, and the unavoidable consequence of having so many young and/or inexperienced cyclists in such a small space.
A last thought. For the most part, the road space was two lanes wide. A single lane of motorway can accomodate 2100 cars/hour. So this road could have accomodated 4200 cars/hour, perhaps a bit more at slower speeds. Skyride had 85,000 participants, so the peak flows would have been what, 20,000 maybe 30,000 bikes/hour? That should give you an insight into how inefficiently we use roadspace in central London, when we allow almost total domination by motor vehicles thereby scaring would-be cyclists off the road.