Wenchy, it's not some sort of irrational blind loyalty, but a recognition that certain parties represent certain core voters and ideologies that aren't going to change overnight. Unless they're incredibly naive, people don't go out and vote BNP based on the strength of their election manifesto, because they know their hidden agenda is the destruction of democracy and civil liberties. As we have seen with New Labour, for all the Tory-esque policies on crime and foreign affairs, they have still pursued the core Labour agenda of tax and spend and the welfare state. Likewise, with the Tories, they can't be trusted not to look after the rich and the English home counties at the expense of the rest of the UK, with all the division, despondency and crime that accompanies.
As far as I am concerned, the party who will look after my financial interests are Labour, but being ideologically libertarian I can't bring myself to vote for a party that treats its own citizens like naughty children, who need to be told what's best, put on a database and spied on.