There is a good reason why people who kill with cars get shorter sentences. It is because no specific malice is involved only recklessness. The maximum sentence has been recently increased from 10 to 14 years.
In this particular case from the looks of it he may have been under 18 at the time and the law is more lenient with jeuvenile offenders
Manslaughter has a life maximum,
It seems that the driver must be "grossly negligent" to be charged with this. However drivers almost never seem to be charged with
The sentencing guidelines investigation
http://www.sentencing-guidelines.gov.uk/docs/d eath-by-driving-advice.pdf are quite interesting, they point out this although they have no influence over charging decisions.
I don't think there's any need for tougher sentences for either of these crimes, what seems needed rather is for a review of charging policy - so that appropriate cases are prosecuted for manslaughter and not the lesser offence.