My own guess is this:
1. The producer in question must work for Top Gear's production company, not the BBC itself. If he'd worked for the BBC you can bet the farm he would've reported the incident right away.
2. "Long day, no catering" must translate into "producer of segment actually forgot to arrange food for everyone when they were done." That plus the stress of the day would have made JC lose it.
3. The real question is whether JC went into a "put'em up" position. The fact that police haven't been called in suggests that everyone involved regarded the "fracas" as minor, but if JC actually connected with a punch that's grounds for an automatic suspension right there, popular presenter or no.
4. The length of the suspension becomes an issue. Remember when Hammond had his injury? Far more serious than this and TG was off the air during that time. If JC's suspension is for a year or less, then TGUK in current format can take that time off and, after the opening addresses the issue, come back.
5. If JC decides to leave permanently, then the BBC has a problem. The TG franchise can be kept going in the short term with DVD sales, downloads and other licensing, but it brings in too much revenue via BBC Worldwide to be ditched altogether. Thus, there would be a need for a replacement. (RH and JM would probably be able to continue on the show for at least the duration of their contracts.)
6. Purely on a temporary basis, Andy Wilman himself could help present the next series; he's always been a part of the production that went out in the field for their adventures and is most familiar with the format. (And don't forget that he himself used to be a Classic TG presenter.)
7. While there are plenty of potential replacements out there, the eventual one will necessarily have a different storytelling approach to JC, and the format will have to be adjusted accordingly. And nobody, at this stage, is qualified to say whether such a replacement can be a success. (Personally I like the idea of them porting TGUS episodes to fill the gap.)