But the Tardis wasn't originally like that! The chameleon circuit broke after a short-out (I think) and it was stuck as a police box unless the Doctor fixes it, which he said he won't because he likes it. Yes, welcome to the nerdy world of me.
Not to put a sonic spanner in the works, but aren't the cover pictures suppossed to be a cutaway on the Haynes? (To be honest I was googling 'Haynes TARDIS' just to see if anyone's managed to pull off a cutaway of a dimensionally transindental machine.)