Not cricket

Various incarnations of the Doctor, as well as some humans, would call a thing "not cricket" when they wished to indicate it was unsporting or morally incorrect. Commonly, the expression, "That wouldn't be cricket," was employed.

Soon after she joined the Fifth Doctor in the TARDIS, Peri Brown once suggested time travel could be a way to make a "killing" on "Wall Street". The Fifth Doctor immediately upbraided her, saying with an appalled face, "Really, Peri. That wouldn't be cricket." (PROSE: The Ultimate Treasure)

Likewise, the Brig told one of his men, Sergeant Mick Houghton, that a full-frontal assault by their then-current enemy "wouldn't be cricket". (PROSE: The Devil Goblins from Neptune)

As a young second lieutenant in the Royal Navy, Lethbridge-Stewart once participated in an inter-ship cricket match, in which his side was helped by Hermy, an Immortal. Indignant at the revelation, the lieutenant threatened to tell the umpire because being helped by an "invisible god" like Hermy just "[was] not cricket". (PROSE: Deadly Reunion) Recounting the story of a woman named Celia who had taken her man's Browning and kneecapped him when she discovered him in bed with another woman, Mark Barrington once told Geoff Paynter that Celia's actions were "not cricket really". (PROSE: The King of Terror)