lol stop whining mate, the rules are the rules. Speed limits also enforce better fuel economy. Not sure about the UK, in Spain they were enforced in the 70's during the petroleum crisis.
There is also people whose driving skills are crap. You may drive like Schumacher at 100mph but other people can't and try to, resulting in deaths galore - driving too fast on bends in B roads, driving too fast in puddles etc.
In town, driving over 30mph is mad. People have cats, they lurk the streets. Kids may appear suddenly on the road going after a football. You may go over a blind bend just to find a child on a bike.