Tunnel from Catford. I think that is fantasy land it would cost the earth and this area not ideal for tunnels ( that is why few tunnels south of the river ).
About 20 years ago they were thinking of building a tunnel from The Grove Pub on Lordship Lane to Stanstead Road to take South Circular under Forest Hill. Was a great idea but did not get any further because of cost and problems in tunneling with local soils.