The Britischer Freikorps (or BFK) was an organisation that policed Great Britain in an alternate timeline in which Germany won World War II. Their authority stemmed directly from the Third Reich and superseded that of the civilian police. Although nominally a law enforcement agency, they were little more than thugs who extorted and beat whomever they wanted.
The Seventh Doctor and Ace were arrested by Freikorps officer Anthony Rupert Hemmings shortly after arriving in this timeline. After they escaped and found the TARDIS missing, they returned to Freikorps HQ where the Doctor posed as the Reichsinspektor General to get the help he needed to get back his TARDIS and find out when the timeline had changed. (PROSE: Timewyrm: Exodus)