I agree it’s complex, but surely, the fact that you’ve been provided with a hire car to replace your’s is the limit of what their insurance company would be expected to do (i.e. – you have not been put in a position where you do not have a car)? For the insurace company to insure your ‘van as well, which has nowt to do with the claim would be generous of them. I can see that you have, perhaps, been inconvenienced, but surely the insurance co have done all you could reasonably expect them to do?
EDIT: or put it another way, do they know you have a van and want to use it/have you told them/what did they say, and why?