I work in finance. Yes, it's true... there's generally a preference for articulate, polished and confident people who basically 'look the part'. It doesn't matter whether they come from private schools or state schools... but it seems private schools churn them out more easily than state schools.
Saying that though, at my current work place the two guys I sit closest to are products of major public schools. They are nice, funny and down-to-earth guys just making a living and couldn't give a toss where people are from or what accent they have.
There's a guy on our floor who obviously went through a different route, and has a very strong accent that in movies would always belong to someone from a lower class, if you know what I mean. The two public school-educated guys never hold this against him - they just treat everyone the same.
However, there are a few guys (and with 'guys' , I literally mean 'men'... there's really a lack of women) who've gone to a local comp, have less wealthy backgrounds, etc. ...and they all have this chip on their shoulder that they were not given the same opportunities as the public school boys. But when confronted with the guy with the strong accent, I'll repeat what many of them think and one of them actually said out loud: "Why on Earth did he not learn to speak properly? He might know a lot, but when he talks, he just sounds stupid and uneducated!"
My point is - it's not just a case of 'old boys' network' anymore. Even people without the 'posh' background will sometimes prefer those who had one to those who haven't had all the privilege because of how they appear.