Police print out a criminal record in a common format so it's easy to locate the information. That is, it has a predictable form (arrangement), like any well-designed form (document with consistent placement of specified information).
Thus, form becomes a synonym for record. To have a form becomes shortened to have form, much as to graduate from a university has been shortened to graduate university (which violates the meaning of graduate, but that deserves a different soapbox).
When a suspect has form, the record establishes a pattern of behavior. Thus, to have form, generalized, becomes a metaphor for having a history of certain actions.
From Death in Paradise, season 7, episode 7, Dark Memories:
DI Mooney: And he has form [a criminal record], right?
DS Cassell: Two charges of theft and one for aggravated assault.
Officer Hooper: We've got a guy in custody with proven form [pattern of behavior] and he's confessed to the murder.
Britain and Germany have a little form over the past century, then, means that the two countries have a history of interactions following some pattern.
Little qualifies form with an ironic understatement of the degree of the pattern. In other words, the author is saying, Britain and Germany have a considerable pattern of interactions over the past century.