Based on this thread, it appears that you still don't have an answer to your question regarding the knives that Bear Grylls uses on Man vs Wild. I say "knives" because he doesn't necessarily use the same knife on all episodes of the show. For now, I can only give you a definitive answer for one of the knives he has used on the show.

It took me a while to figure this out, but I finally found the knife. On the "Moab Desert" episode, approximately 6 minutes into the show when he's cutting his white t-shirt, it's clear that Bear's using a 3.5 inch Buck Iceman Tactical Folder, model 873X.

I don't necessarily think he's using the knife because he likes it. I suspect that he's using that knife because the folks at Buck paid for the product placement. If it was product placement, however, it wasn't done very well.