Dennis, have you lost the ability to read a map or sumpn???
I've just bought an '03 XK8 and was told by various people that the built in satnavs were not reprogrammable though that may not relate to your later model. The various owners fora are loaded with info on such subjects.
I'd consider removing the useless thing and getting a modern unit mounted behind the dash in its place. All you'll see is the screen, no one will be able to tell the difference plus it'll probably be a damn sight easier to use.
I have a tomtom with the cable led behind the dash, thru the centre air vent hose and just the plug showing on the passenger side of the grille. There's enough slack to plug it into the unit when I need it using an air-vent mount. When not in use the slack slides back into the air hose and the head lives in the armrest that the daft motorola telephone handset vacated on it's last journey (to the bin).