Just to be pedantic, the offence is one of "Careless Driving". Care (whether due or undue) does not come in to it.
To answer your question mrtfool, the circumstances you describe most certainly do indicate that careless driving occurred. It was not an accident. Your wife did not leave sufficient distance between her and the car in front to be able to stop in the prevailing conditions. Yes, it happens all the time and no, not everybody is prosecuted for it, but it is still so nonetheless.
The maximum penalty is a fine of �2,500 and the imposition of 9 penalty points, or a period of disqualification. The incident you describe is at the lower end of seriousness and the realistic outcome is a fine of about one week�s income (reduced by up to a third if she pleads guilty) and three points.