EXT management promised a route to London last Spring, and then again this year but still waiting. I do not think a route is viable unless it was similar to that operated by ASW (IE Plymouth-Exeter-London) but I very much doubt this will materialise.
With the current focus on the environment, and the potential rise in APD, I have come to the conclusion that despite promises from those in suits sitting in the EXT boardroom this route will not appear as a destination (at least not while you can still use the train!)