18th century: The Enlightenment = science and reason vs. faith and tradition. In England only male property owners (I'm simplifying) had the vote. The two main parties were the Tories - faith and tradition, economics run by the Crown - and the Whigs, progressive free-marketeers, including what we would call lefties but also very rich property owners.
19th century: the vote extended gradually to all male citizens. The Tories became the Conservative party, and the Whigs became the Liberal party. The party division on economics shifted, as old style monarchist centralism became obsolete, with the Liberals increasingly soaking up the votes of the urban educated and progressives, and the Tories representing landed interests and deferential working voters.
Into the 20th century the Liberals started to reflect the interests of the working classes, bringing in Old Age Pensions, extending Income Tax and limiting the power of the House of Lords. Women get the vote (hurrah!). The new Labour Party got its act together and (with many hiccups) became the main left-ish party in the UK. The main division was between the Conservative/Tory party on the Right and the Labour party on the Left. The Liberals were left to represent the anti-socialist but progressive sector, the celtic fringes, and a few other atypical consituencies; they had few MPs.
In 1979 Thatcher got in, things changed a lot. From this point on opinion is liable to be stronger and more divided. Labour temporarily became more left-wing (arguably). Right-wing defectors from Labour were soaked up eventually by the Liberals (I'm simplifying) to make the new Liberal Democrats. Now Tony Blair has moved Labour further to the right, and some left-wing defectors have also moved to the Lib Dems.
Of commentators on recent threads, iainjcoleman is standing as a Lib-Dem candidate in this election, and believes that they represent the future of progressive politics in the UK. I disagree on means but not so much on ends. Other brits on my friends list are Tory supporters, Labour supporters, Greens, Socialsts, etc.