Why doesn't Parliament fix this?

[Prosecutor] Lt Col [William] Peters told the court martial board, which acts as the jury: "The reason this is now being heard at court martial before yourselves as opposed to a jury in the crown court is because these events are alleged to have taken place in Germany therefore outside the jurisdiction of civilian courts."

From this account of the court-martial of a British former warrant officer

Quaere: why not fix the statute so individuals in this accused's position can be tried in a civilian court? Human right jurisprudence strongly disfavors the court-martial of anyone other than serving personnel.