What does "to have a little form" mean?

ODO on form

The relevant entry is 7c; none of the others really fit the context:

7 [mass noun] the state of a sports player or team with regard to their current standard of play:
they are one of the best teams around on current form

• details of previous performances by a racehorse or greyhound:
  an interested bystander studying the form
• a person’s mood and state of health:
  she seemed to be on good form
• British informal a criminal record:
  they both had form

In OED it’s moved down to 16c:

c. slang. (Without preceding article.) A ‘police record’; a criminal conviction.

In this case it doesn't actually mean “a criminal record”; it means “a history of criminality” or “a history of conflict against each other”.

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.

(later)

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.