Regarding the South Downs, you could always jump on a Thameslink train down towards Brighton.
If you get off the train at Hassocks then you're at the foot of the Downs. A quick slog up to the top of the ridge and you're on the South Downs Way. From there I'd recommend checking out Stanmer Park, a mile or so south of Ditchling Beacon, which is to the East of Hassocks.
There's tons of singletrack in Stanmer that's looked after by local riders. All unofficial/unmarked paths though but you really can't miss it if you keep your eyes open. From Stanmer you can fit in a loop back to Hassocks and back to the station.
If you wanted a longer ride perhaps get the train down Eastbourne and ride the South Downs way back towards Brighton. Something like about 35 miles or so but a nice ride.
Just a couple of suggestions. Cheers.