Paid Ride Marshals and Leaders Required, SW London a priority

Also...from tfl website, in that lovely tiny font of theirs:

Taking part

All guided cycle rides will take place subject to adequate demand. To join in, please email us at smartertravel@tfl.gov.uk with details of your preferred ride route, date and time. Two days before the rides takes place, you will receive an email to confirm your place. If there is not enough demand for the ride, you will receive an email letting you know that the ride has been cancelled and where possible, details of alternative ride dates and times.

and from email from Lu Everett at LCC (communicating on behalf of TFL I guess):

There will be a huge press and marketing campaign for these rides, called **[B]‘Ride with a Guide’ **[/B]which will start on 19th July.
which I haven't seen/heard too much about.