It depends on how the evidence is presented - the person saw you overtakjing so slowed to assist.... or the person saw that due to your overtaking an accident was likely to occur and slowed to prevent it... as I said, make sure you get proper advice before giving evidence.
Also the highway code isn't law - it quotes law, and can be used in proving offences, but if it says SHOULD the it's good advice, whereas if it says MUST then it's referring to actual legislation. Breaches of the highway code can be used to back up evidencem, but the overiding issue is that your vehilce, by overtaking, put itself in the path of on-coming traffic. It's then for the court to decide whether the incident could have been foreseen. If it was a long stright bit of road it will be viewed very differently to a narrow winding road. Also bear in mind that a speed limit is a limit not a target, and markings on the road indicate that it's lawful to overtake - not that it is safe to at the time - the onus is on the driver to drive safely in the conditions. An extreme example would be on an unrestricted road in heavy fog and ice. It may be lawful to drive at 70mph, it may be lawful to overtake at that point, but it wouldn't be safe.
I knwo I've already said it, but take legal advice prior to legal challenges etc - everyone has heard of the highlighted cases where people have been found not guilty by cleverly challenging evidence and procedures, but the vast majority of people who try that route wihout proper experience fail and can sometime end up with more serious penalties.