Your assumptions are incorrect, you will have to pay for it

As said. It was not advertised to you as having a towbar, and it was most certainly not a deciding factor in your purchase. As such, you have no rights to make any 'demands' over it.

As others say you discovered car has tow bar basics, presumably including electrics, already fitted. You're just missing the detachable bit which, if you cannot get it from previous owner, you could probably source from a caravan dealer or towbar specialist such as Witter.

Cost me £700 to get a dealer fit detachable towbar and modern single electrics on my Berlingo. Even if the cast steel swan neck bit costs you £200 you're quids in compared with a standard car with nothing.

As to dealer providing you're in a dream world.

Most 2nd hand buyers are probably more concerned that regular towing of something heavy has put extra strain on the car: hence visible part of expensive accessory is removed, probably by previous owner rather than dealer. But I don’t think I’ve told anyone anything new.

And I don’t think that the dealer is responsible either.