I was also looking at the flip up style single din stereo for the XJ. Could not find a model that did not interfere with the top of the dash. So, i looked at fitting a double din stereo. I wanted one with all the fruit. Reversing camera, gps, dvd etc..
You need 100mm of height to fit one in. With the pre-update XJ, there is no way to get a double din to fit without some serious dremel work.
You have to create a cut out in the central air vent so that you can drop the control panel down. The reason you have to create a void in the lower vent is because the contol panel has a lump of plastic under it which will not allow it to go down far enough unless you make the cut out.
You can use a dremel to cut out the vent and then make a small drop down box to insert.. then use aluminium tape to seal it all up.
You also need to reduce the size of the vent tube at the back of the void. This tube sends air to the windshield. This can be cut down with the dremel and aluminium tape applied to reduce its thickness. This gives you the depth to fit your double din stereo.
So, you have made your drop down tray, taped it up, cut back your vent tube and taped it up, fitted the control panel in its new lowered position and it should all look roughly like this...
You can then wire up your double din stereo and fit it like the one below.
Once its all fitted up, you will have to use your dremel to slot out the facia panel to suit.
And there you have it. Its not easy but its a mod i always wanted to do...