For the offences you mention the maximum penalties are as follows:
Careless driving � a fine of up to �2,500 and either penalty points (minimum 3, maximum 9) or a disqualification.
Falling to stop/report (which, in the circumstances you describe would be only a single offence) a fine of up to �5,000 and either penalty points (minimum 5, maximum 10) or a disqualification.
In general, where a number of motoring offences are charged which result from the same incident, magistrates impose fines for each offence, but award points or a disqualification only for the most serious.
The likely outcome, therefore, if you are charged with two offences, is a fine for each and between 5 and 10 points for the failing to stop/report. My view is that, if so charged, magistrates would award the minimum of 5 points for the most serious offence. From the way you describe the incident, the offences would be at the very lowest end of seriousness and so would attract the minimum penalties.