The Chilterns, I think, and burying the line in tunnels was more or less necessary anyway. Some of the tunnels could technically probably have been replaced with cuttings, but apparently it would've cost more. The existing line used them mostly because tunneling was a lot harder back then. Mostly, the NIMBYs seem to have forced the existing tunnels to be longer than they need to be, which is expensive but not making building infrastructure in the country entirely non-viable levels of expensive.
