Try On Your Bike
If you click on the individual sections of the route then you get OS grade maps.
Note, if your not doing this with the British Cycling group then I strongly recommend doing it on the weekend as these roads a busy commercial routes and can be fairly tight for space. You'll not enjoy it as much if you are being hassled by trucks.
The section from Westminster to Greenwich is a rolling roadshow for the locals and the racing won't start until until you cross the Meridian.
If you fancy it, there's a good chance that Southwark Cyclists
will be taking part in that roadshow as the local colour.
The train back should be quite doable but do check with National Rail Enquiries
for information about planned engineering works closer to the time, especially if you do this on the weekend.