The London-Rouen cycling challenge consists in cycling from Westminster Abbey in London to Cathedral Notre Dame in Rouen in 12 hours or less.
This is more an adventure than a sport feat : the cycling distance (c.100miles or 170km) is not the problem, the Channel is. There is no set route for the challenge but, in any case, the channel needs to be crossed and this represents a major logistical issue. See Routes.
In addition, if you chose the ferry option, it is highly likely that most of the journey through the French countryside will take place during night-time, which seriously impacts average cycling speed.
Putting aside the race against the clock, cyclo-touring enthusiasts can enjoy the adventure by taking plenty of margin at the start on the UK side, and going at a comfortable pace once on the French side.