I'm not sure about the Discovery, but guessing here from the LR book of cost saving, it uses a very similar/the same loom as an RRC, and behind the RH rear lamp there is a round plug with all the connections for a tow bar in it.
When I had my southdown guard on my RRC, I just used to unplug the socket from here and take the towing equipment with me., Wrap the socket in a plastic bag to stop it filling with mud
Mine was an OE fitment towing kit originally, so could explain why it had this arrangement on the towing socket, but it did make for a very neat solution. I know others have wired in normal 12N sockets, but personally I think it is overkill, and more likely to get broken off road.