Can't see dual-voltage voyagers happening - apart from anything else going over 90mph on 3rd rail is problematic and over 100 extremely difficult (Unless you're a 4-REP running light, re recent posts)
Voyagers would lose the most useful 35mph of their speed capacity through running on 750v. Even without the problem that their traction motors need a higher voltage than that
I have to agree with you, although I'm not sure if they can do 125 on the South Western anyway.
As for starting up on the move, it's probably possible, but most likely not allowed despite the fact that Deltics would sometimes do it on an unofficial basis.