According to the cyberneticist Stafford Beer the purpose of a system is what it does – we shouldn’t look at an organisation’s stated principles, but at its results.
Applying this principle to politics, we can see that for my whole lifetime, the purposes of both the Labour and Conservative parties have been the same – to move that which was formerly the preserve of the private sphere into the public sphere and make it the business of government (ID cards, DNA databases, control orders, ASBOs) while simultaneously moving what was formerly considered the legitimate business of government into the hands of business (privatisations, PFI, outsourcing) in such a way that all the risk remains with the government but the rewards go to shareholders.
The current government hasn’t stopped the second part of this, but thanks to the Liberal Democrats it is partly reversing the first. This is why I can continue to support the Lib Dems despite very definitely *not* being a Conservative. The purpose of the Lib Dems is clearly different from the other two major parties.