Part human, part animal embryos

I have just been reading what seems like an ongoing saga in the UK news. Gordon Brown wants to pressure the government into voting for a bill to allow experimentation on part human, part animal embryos. This conjures up a lot of strange images, not entirely unlike that mouse with an ear on its back. Personally I'm in favour of it, because it has been described as 'militantly atheist' by various religious groups, and I find it amusing that such a phrase is considered a negative comment in this day and age. But seriously, what experiments, what animals and what benefits could come from it? And why is Gordon so keen to do it? Does he have a disease usually associated with llamas that only a human-llama hybrid embryo experiment can possibly cure? Or is it so that the disappearance of educated, employable people fed up with high cost of living and miserable weather in the UK is balanced out by an influx of people keen to crossbreed babies and meercats?