“Prison for GBH s20 is possible but normally only for the worst cases.”
This is not correct. In fact the opposite is true. As I pointed out in reply to your earlier question, only offences at the very lowest end of seriousness will normally result in a non-custodial sentence. The sentencing range even those in Category 3 (the least serious) extends up to 51 weeks custody. Offences in Categories two and one would normally be sent to the Crown Court. I think I provided the link to the Sentencing Guidelines before, but here it is again:
http://www.sentencingcouncil.org.uk/wp-content/uploads/MCSG_web_-_October_2014.pdf
Page 197 of the document (page 191 of the .pdf file) provides the details.
As I also said earlier, it is difficult to place the offence in the relevant category (and definitely impossible to consider the possibility of conviction) without hearing all the facts. But certainly I would suggest that higher culpability is present by virtue of the fact that there was “Use of weapon or weapon equivalent…”. A vehicle may certainly be considered a weapon in these circumstances. This would push the offence into Category 2 regardless of the Court’s view on the level of harm.
The Common Assault charge will not add much damage to the overall sentence. If a custodial sentence is imposed it will almost certainly be concurrent to that imposed for the GBH. It will not aggravate the GBH charge by virtue of “previous history”. Antecedents (previous record) can only be considered once the earlier offence has been sentenced. If both these matters are dealt with at the same time your friend arrives in court with an unblemished record. The court will not sentence for the offence which occurred first and then go on to consider the second (using the first offence as an aggravating feature). Instead he will be sentenced with totality for the two offences being considered. The entire “course of conduct” over a period may be considered to aggravate the offences, but he will have no previous record to take account of.
Your friend needs to heed his solictor's advice. It may well be that the court can be convinced that the offence was committed recklessly rather than deliberately. Indeed the CPS may already have taken this on board with the charging decision. If it was considered he committed the offence deliberately he may be facing a charge under Section 18 (GBH with intent). It may be a long stretch to convince a court that the injury was caused accidentally.