For what it's worth I reckon in-cab signalling will come in in due course depending on finances. It's what they use on the continent (Eurostar etc) and doesn't involve lineside signals or the passage of trains to show up on a signalling centre monitor via the present system (an electric current...
Bi-directional lines have been in place on the BML for many years but oddly is rarely used. I drove Thameslinks for 4 years and went up the 'Down' line only once. The system has recently been enhanced so I suspect more use will be made of it.
Incidently this system is split into three sections...