Following Stevesy's link - whilst there is no doubt that up to two years imprisonment is possible on indictment, the sentencing guidelines are not given, although sentencing case law is. The following are there:
R v Elwood-Wade (1990) 12 Cr App R (S) 51 - D driving at excessive speed, and collided with a police vehicle. Court held that the only aggravating feature was the speed and concluded that in the particular case, a sentence of immediate imprisonment was not appropriate. See also R v Underwood below.
R v Howells [2003] 1 Cr App R (S) 61 CA. "Road rage" cases involving furious driving with intent to cause fear or possible injury, but no accident, consumption of alcohol or injury - six to 12 months imprisonment.
I note that the maximum term on summary conviction (magistrates) is only 6 months - so to get to a year in prison this has to have been seen as at the more serious end.
Previous convictions are an aggravating factor, but apparently mainly where they involve 'bad driving or the consumption of excessive alcohol before driving'. So arguably a couple of SP30s shouldn't have made a difference, though we don't of course have the details of that. However, 'racing' is also an aggravating factor, so we come back to the witness statement, which seems to have been hugely important to this man's personal freedom. My reading of all this is that, given that the death of the other driver was due to his own actions, without a statement that they were 'racing', the DD charge might have been difficult, and certainly imprisonment would have looked harsh.
I still don't know what the law would regard as 'racing'. First away from the lights? This incident apparently only lasted for 10 seconds, after all....