Yeah, where the hell did they get that head from? Its from approx. 30th century. I think they chose that head cos its possibly the most 'classic' looking one. This suggests that the Cybermen obtained that time ship around the 30th century.
You've seen the time travelling theory many times? Damn. 🙁 I've only seen it once on a certain Cyberman fansite and that was after I thought the Cybermen in The Invasion were the predecessors of The Attack Of The Cybermen, who in turn are the predecessors of the unexplainably large and technological advanced force seen in Silver Nemesis (not all of the Cyber ships were destroyed in The Invasion, these made part of the Silver Nemesis force).
Oh and I wont mention it as theory. In my history pages I include evidence for and against a dating. This ranges from 'Date stated in episode' to lesser evidence seen in the story, to evidence against the date (which would obviously not overwhelm the evidence for).
Plus if its not canon, then what explanation is? How can they be that technologically advanced of The Tenth Planet Cybermen? It needs an explanation and I think this one's a pretty good one. Just asking which one you prefer, or if you have another explanation, for fun, since The Invasion came to mind. 🙂