Ride along Brighton seafront (there’s a cycle lane) as far as the marina. Continue straight through the marina leaving Asda on your left. At the end cross the bridge over the lock and turn right towards the sea wall. Take the path behind the chandler’s inland towards the undercliff path, then swing right and follow the path west. This will take you on a flat traffic-free path by the sea all the way to Rottingdean, about 3 miles or so.
Unfortunately you then have to rejoin the main road, but it helps make the most of Brighton.
Courtesy of Jonathan