I've had many commerical insurance policies over the last 30 years.
So far, not one of them has covered me to drive another vehicle but they do exist - I've just never found one.
My car insurance does cover me to drive another car that doesn't belong or is hired to me - but this is only a CAR not a van or any other commercial. That's another exemption to be wary of.
As always the detail is in the fine print, which you have to read very carefully.