The software on the BT box *is* the standard software although the release schedule may sometimes vary as to when an individual unit gets the update as happened when BT were prepping their boxes ready for the launch of BT Sports last month. Retails boxes got those changes later (albeit confusingly with a point update in the version number).
If you are not on BT broadband and connect a BT box it will change to the retail version.
These boxes are locked down and not intended for the inveterate tinkerers amongst us so the chances of any users mods are vanishingly small.