Well it wasn't stated as a scientifically testable fact, but centre-left parties have been getting elected and then swinging to the right for quite a long time now :P Certainly polls and discussions here on this forum in the past have shown most of the Brits showing a left of centre, slightly anarchistic political disposition.
Out in the real world the only conservatives I meet are business owners or Baptists (or other Christian denomination, I live in Northamptonshire though so mostly Baptists) - which combined would be less than 25% of the population.
I realise that Conservatives gain votes from elsewhere, but I think that is because so many people think all politicians are about equally evil and swing their vote based upon how popular the current government is with the press.
So whilst I wouldn't say that my original statement was absolute fact, I do think there is a sizable proportion of the population who would fit that description - and my original point was that this demographic - despite its size - is not represented in politics, because everytime we vote people in who claim to represent us they swing to the right like a lead filing to a magnet.