Notice:
- All bricks that the interior is made up of have some kind of effect (chrome, blink etc)
- We have not seen a shot of the tardis from above with the doors open
- The interior stuff seems to dissapear in when the door is opened.
If you place some bricks with almost complete transparency and turn shaders on, you won't be able to see bricks with effects or transparency through them.
Not to say that this isn't a really good way of doing it.