Thanks for the picture - those 'in the know' will be able to guide you ! As I understand it, you have a choice of the "Silverline" type that sits inside the hitch, so prevents the tow ball from being attached, or the 'Cover type' which prevents the release handle from being lifted, thus preventing attachment to any tow-ball ! Sure there's more advice out there in Eribafolk land !
Glad you got a good insurance quote :)