There is National Cycle Route 21 (aka Waterlink Way) which crosses Beckenham Road near Penge (near Kent House Station) and then heads up on a traffic free path mostly next to various rivers (so it is flat!) and when you get to Deptford Bridge you could then go left towards New Cross and Old Kent Road. I don't know of any recommended cycle route from Deptford to the OKR, but you could ask again on the EDF and see what gives. Certainly NCN 21 is flat in the section I have mentioned, and avoids all hills.