Why does Humanists UK campaign against creationism being taught as science?

Because ultimately, it is not based on scientific method. If it can't be empirically tested in a lab - which it can't be, because it's based on religious ideas - then it isn't something they think should be in a science lesson. There are other reasons too - Humanists UK are opposed to the teaching of all religious ideas in schools, on principle - but that's the key reason.