There have always been a disproportionate number of successful Scots in UK politics, the media and in other areas. This might be because they are more ambitious, or perhaps Scottish education is better, but it might be for another reason that is hiding in plain sight: the Scottish accent.
A politician with a north of England accent is judged as "common" by middle-class southerners, and a southerner with a home-counties accent is viewed as "posh" by northerners (and probably working-class southerners too). However, a modest Scottish accent is seen as neither posh nor common and therefore cuts through the class prejudice.
I can not imagine a politician such as Charles Kennedy becoming a major party leader if he had such a strong accent but had grown up in Birmingham, the West Country or the East End of London. Only Scottish, and perhaps West Yorkshire (ie Harold Wilson), are accents that command respect from all English people.
Getting promoted in middle-class cliquey professions like medicine and law is probably easier if one has a Scottish accent as it will not suffer snobbery that, say, a broad Mancunian, Scouser or Geordie would likely encounter.
The accent of TV presenter Stephanie McGovern - who is from the north east - was commented on quite harshly by snobbish types, whereas the long-standing proliferation of Scots on TV has never been mentioned.
Will "Scottish" remain this way forever? Perhaps greater devolution across the UK will encourage the English to have more pride and appreciation of the amazing diversity of this tiny geographical country and start to value ALL English accents.