As some of you may know, the first police call boxes which look like the Doctor’s TARDIS were initially placed in Scotland in the year 1929. Doctor Who premiered well later in 1963.

The British Broadcasting Corporation, known as the BBC, applied for a trademark on the police boxes in 1996, after they had been phased out from real life usage.

A couple of years later, the Metropolitan Police challenged this application, claiming that the trademark belonged to them. However, since no police agency had ever filed a trademark, and the public only saw police boxes in Doctor Who episodes, the BBC’s application went forward, and the Metropolitan Poilce was ordered to pay the BBC £850 plus the legal costs.
And this explains how an everyday object (well it used to be) became trademarked by the BBC. And you can’t go put a police box on a t-shirt and sell it legally.

VIA io9