The police have the option of throwing him out on his ear without a pension, and with a big black mark on his CV.
In order gain a conviction on charges of murder or manslaughter, the prosecution has to prove beyond all reasonable doubt that the actions of the accused were the sole cause of the death of the victim. In this case, there were at least two conflicting post-mortem examinations, one of which was produced by a doctor whose work has come under query. The alternative pathologist apparently thinks that the assault may be a contributing factor - not the same as the sole cause. This renders it probable that there would be two conflicting sets of evidence, each proposed by a leading QC, with the jury (who are not forensic experts) having to decide which they prefer. This probably had a big bearing on the CPS deciding not to continue.
There is a possibility of the inquest deciding what was the most probable cause. If you remember, during the Menezes inquest, the coroner barred the jury from bringing in a verdict of unlawful killing. If the coroner allows the possibility of any verdict, and the jury decides it was an unlawful killing, then PC Harwood might yet go on trial. There is no statutory limits on homicide (ie murderers and manslaughterers can be put on trial at any later time).
The biggest blunder by the police or the CPS was that PC Harwood was not charged with a serious assault shortly after the events happened, as soon as PC Harwood became known to have been involved. As a consequence, the chance has now gone.
There would also be an argument about the validity of a video shot on a mobile phone. You will also remember the case of Kate Moss, allegedly seen on video snorting cocaine, because there was room to doubt that it really was cocaine in the absence of any positive drug test evidence.
In this county, firearms officers go around in that type of vehicle. They are not trigger-happy in Lincolnshire. However, in Nottinghamshire, they tazered a teenage girl by accident during the week.