Exterior: when the Tardis health is below 20%, the exterior will change to a slightly damaged version and at 0% it has a very damaged tardis shell.

Interior: when the tardis is on 20% health or lower, the monitors could have a WARNING alert on them in big red letters and possibly cracks on the screen.(cracks would be part of the warning image.) when the tardis is at 0% health, could the console monitors flicker on and off with loads of cracks in the screen, and could the monitors themselves be falling out of place as well a one of the contol panels like in "journey to the centre of the tardis". (if that is possible)
when the Tardis hits 0%, before everything in my last request happens, maybe there could be a burst of sparks and the sound of an emergency shutdown.