Lord Archer's history includes a lot of VERY questionable people, EXTREMELY dubious activity, a deliberate attempt to conceal this from the proper authorities, and an attempt to pervert the course of justice.
The driver, on the other hand, showed a slip in judgement and some negligence. Yes, he killed a child, as a result, and that cannot be ignored. I honestly doubt the man will ever forget that. If he uses the experience to become a better person, then to punish him further would serve no purpose.
It is for reasons like this, that I firmly believe that a justice system centered around vengence will punish the careless and free the dangerous.
It is also for this reason that I now believe that the entire justice system, as implemented by the US and the UK, is fundamentally flawed, with the US system the worse of the two.
A better solution, IMHO, is to argue that, in both cases, there is an almost childish lack of understanding of consequences. In Lord Archer's case, I'd argue that there is also good reason to believe that sexual addiction plays a significant role. If you accept that at least some apparently "normal" people are, in truth, emotionally ill, and that dangerous, even deadly, consequences can arise, then treating that illness makes much more sense than punishing the person for having it.
Sure, incarcerate Lord Archer for four years, or so. But given the choice of a sane Archer emerging, or a crazed, vengeful, psychotic one, I think the saner one would be preferable. Don't you?