Ah would it be related to the CD changer in my boot? Pic below.It's coax, so if the car radio is working ok ( probably not the standard aerial cable as it's not usually routed that way ) it's probably the remnant of some aftermarket DAB/GPS system.
There is a DAB aerial attached to the inside of the windscreen that I assume the current head unit uses. I don't particularly like it being there!!! I didn't even know what it was when I bought the car, never seen one til now.As there was DAB in the car, my inclination would be that there was a splitter/amplifier near the aerial and a DAB feed was run from there to the head unit. That may be the one that has been cut.