Grab a good detailed map of paris like a Michelin (they should have those in London right?), and print out the detailed Itinerary from lelour.fr for the stage. With that and a map, you can figure out the route pretty good. I've done that a few times no problem. It gets a little complicated in the close suburbs.
If you want the streets in the city limits that lead up to the circuit they ride, get there real early and go for a walk up la rue de rivoli. The cops are usually out and the route should be marked and it's pretty easy to figure out how they are going to enter paris.