I was under the impression butterfly knives, as well as switch blades, were totally illegal to even own in the UK. Like Martin says, they are designed to be concealed weapons, that can be deployed in an instant, unlike commando knives, bayonets, survival knives, chefs knives, etc, etc,...
It is totally illegal to buy such knives in the UK, as well as sell them, so it makes sense they don't want us owning one. I would seriously check the "actual" laws (rather than what people say) if you own one, to make sure you are permitted.