Battlefield - S26-E1
Character mistake: The Brigadier asks the pilot if she speaks Czechoslovakian, even though Czech and Slovak are different languages.
Continuity mistake: When the Doctor picks up the humanoid skull his hat is looking normal but in the next shot the brim of his hat is folded down and completely flat. In the next shot it's back to not folded again.
Answer: TARDISes are generally available for properly authorised use on Gallifrey; they're not usually assigned to a particular Timelord on a long-term basis. The Doctor stole his when he left his homeworld.
Tailkinker ★