Rosa

[kingpin]

AWJ-1 again.

And from the "Next Time" segment:

 

[ciderman]

Is that a special 'slanted' base they've used to keep the TARDIS level?

 

[kingpin]

Yep, and it isn't the first time we've seen a modification like that.

 

[angelus lupus]

Actually, going by the overhead shot, it's a thin plank/slat along the front to hide whatever wedges they used to level the box:

 

[kingpin]

D'oh, and it's so painfully obvious.

 

[ciderman]

Yep. Sticks out like a sore thumb! Would be more realistic just to have it at a slant, or hide the fake base with some alleyway rubbish. 

 

[galacticprobe]

Would be more realistic just to have it at a slant...

Yup... just like they did during the Classic Series. If the ground was uneven, so was the TARDIS when she materialised (provided she's not so uneven that she's in danger of toppling, that is).

Dino.

 

[kingpin]

Would be more realistic just to have it at a slant 

If anything, surely this effort... As shabby as it is, is more accurate to real Police Boxes.  There's more than a few (on the more uneven locations) where the bases have a uneven plane due to the local topography, but presumably ensure those Police Boxes were as dead straight as you can get.

Plus there's the stress factor to also consider, the prop is designed to stand up straight, letting it lean towards one side or corner or another puts pressure on the various joins and fastenings, and that may be the one of the reasons why some of the classic props deteriorated they way they did.

 

[angelus lupus]

I see it as part of the Tardis safety features: slight slope? adjust the base to compensate!
Of course, severe slopes like mountainsides can prove tricky  ;D