The examples that you have cited are all rich or at least upper-middle class people to begin with, even before they entered politics.
The "idealism" being discussed here at this juncture is, can a lower working class man be an MP as long as he has the right "value".
And what value is "right"? Who's to judge?
I wouldn't know. But I believe that even a future PM-in-waiting, if any appears to challenge, then he would probably have to be english-educated to a big extent, and possibly belong to the middle class or the upper middle class.
Still what type of "right" values are up to one's perception. However the world is changing and often more than not, a top leader will be needed to be seen as a credible choice so that he can deal with problems that are international. I mean, understanding economics is a big criteria here, so you wouldn't expect a guy who knows zilch about economics to be PM, wouldn't you? This means that a potential PM needs to learn economics, law and needs to understand global warming, trade and other stuff. Its not a fly-by-pants job that anyone can learn OJT. No offence, but certainly, one who can be PM cannot be one who's completely chinese educated, or one who speaks dialect only and knows absolutely nothing about economics. The odds of such a guy becoming PM is very high and against him.
Thus, I think that as long as a leader can make that special connection, and not be cocky, at least publicly, then he or she will be able to make a big difference in the future.
Besides, Sen. Obama wasn't born to a upper-middle class family. He was born to a single mum. It was through scholarships that Obama was able to attend top US schools through grade school. The same is said for both Clintons, especially Bill, who shares a similar history like Obama a generation ago.
And FDR and JFK, although rich and wealthy, was able to make that special connection with ordinary people, despite their status. It speaks volumes about their overall ability even when they have a handicap because they were born to well-heeded families.