Wow! I knew that if there was no seatbelt fitted for a seat, then you didn't have to use one, but I had no idea a company could hire out a vehicle with insufficient seat-belts.
To my mind, it's very different if you own it ... then it's your own stupid choice ... but if you rent you kind of expect to have minimum modern safety standards.
A quick google seems to suggest the hire laws are even more ambiguous than the seat-belt laws!
I would bluff.
I think you should contact the company, tell them that you have now been informed that, yes, they can indeed offer for hire a vehicle with insufficient seat-belts for the number of passengers but only if they tell you that there will be insufficient seat-belts for the number of passengers, ahead of the hire.
I think you should tell them that you are consulting with trading standards, and that you require either a different vehicle with a minimum of four seat-belts; or a fourth seat-belt to be fitted to that vehicle ahead of your trip; or a full refund.
(I think you should probably put this situation to your local trading standards office as well, to get their take on it, just in case they can give you some back-up, which supports the bluff).
If the bluff fails and you get no where in time, I would take a second vehicle. Yes, extra fuel costs, and you're not all together ... but it can still be fun driving in convoy and you can park up the van at destinations, then use the car for local travel.
No way would I do that journey without a seat-belt, especially in a side facing seat (which I find harder to maintain my balance in).