Addition 01-11-11: John B commented that unions are legally obliged to offer an opt-out on donations to political funds, however see my comment in reply for an example of how this law may be abused.
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This follows on from my previous rant about how manifestly evil the UK Labour party is. One of the points I forgot to make in that rant was about union funding of Labour, as revealed in this Graun editorial. Rusbridger describes how legislation to cap donations to political parties- a manifestly just and sensible idea (and another Labour failed to action)- would also hamstring union funding of the Labour party unless the political levy unions hand to Labour were ended and individuals were encouraged to donate themselves. Yes, you read that correctly. Regardless of the political alignment of the union member, who may very much need the protection and security afforded by membership, he has no way of preventing the union handing over his hard earned money to a political party that has- to say the least- taken a big, sloppy, wet shit all over the people of the UK. Interesting, yes? But wait, there's more: Rusbridger goes on to make the astonishing observation that, when levies were devolved to individuals in the past, the Labour party saw "big falls in the levy". What does that tell you about how much genuine support the UK Labour party has among workers.
I was, in fact, aware of this before I read Rusbridger's piece. I signed up to join Unite when I started my first job (which was also my PhD) after completing my Master's. Being the good little ideologue that I am I proceeded to crow about this on the internets. My bubble was rapidly burst by Rossinisbird pointing out that Unite actively lobbied the Labour government for new nuclear builds. So I cancelled my membership within a week. Fuckers.