The Financial Times said the 19 judges went through a mock trial, designed by the International Bar Association (IBA) on Wednesday to replicate the circumstances they might encounter at the Iraqi Special Tribunal.
The exercise represented the final leg of a week-long training effort being funded by the British and Australian governments, the daily added.
"The issues are daunting, with regard to evidence process, the protection of witnesses and the rights of the accused," Mark Ellis, the IBA's executive director said.
The assembled judges first paid tribute to a fellow judge killed in Baghdad the previous day, before getting on with the rehearsal.
Investigative judge Barwiz Muhammad Mahmud al-Marwani and his son Aryan Barwiz al-Marwani were shot dead on Tuesday as they stepped out of their Baghdad home.
The judge was the first known member of the tribunal to be killed.
With hotel chairs and tables shifted into place and an Arabic translator filling the role of a war criminal, some of the judges acted as defence counsel while the others sat in judgment.
They explored a series of crimes, including murder and persecution, committed in a fictitious country called Alpha, using the laws and procedures that will be applied in Iraq.
Stuart Alford, a lawyer who worked as a prosecutor before the special panel for serious crimes in East Timor, said the exercise exposed the judges to "day-to-day judicial work" and familiarised them with the procedures of the penal code.
Questions raised by the group during their time in London ranged from big legal questions such as sovereign immunity for former heads of state to more technical matters such as witness protection.
Due to the threat to their lives for working on the tribunal, the judges with Kurdish and Arab-Iraqi backgrounds, have kept a low-profile during their London visit.