I'm up for doing this as I have before, so cycle down to Newhaven with the crowd making their way to France, have breakfast, wave the Francophiles off, and then cycle onto Brighton, and get a train back to the Smoke.
Admittedly the route to Brighton isn't exactly the most quiet road from a traffic point of view, but at least the final bit from the Marina along the sea front is relatively motor vehicle free. There's then the option of grabbing a cup of tea from the Madeira Cafe or continuing into Brighton for somewhere more salubrious.
(I think we're still just about OK to get a train back, and the Olympics ban on bikes hasn't quite kicked in at this point).