The bloke pleaded to causing death by dangerous driving and was sent to prison for his gross error of judgement. The law on dangerous driving is such that I don't think he had much chance of defending the case, but, as it goes, he was a decent person who was feeling incredibly guilty about what had happened wanted to put his hands up at the first opportunity.
Speeding by itself does not amount to dangerous driving and doing 45 in a 30 would not be prosecuted as such. Even if speeding alone was sufficient, you would struggle to find a jury who would convict simply because a competent defence advocate would slip into their minds the notion that just about everybody – including them -speeds at some point.