My interpretation was that Mo would have revealed the truth on the stand - as he had been shouting his mouth off in the previous episode - that his father had been working as a 'tout' for years. This would have outed his contact - the MI5 agent he was dealing with - and blown the whole long-running operation out of the water.
Tina met the agent urgently and warned him that if the trial went ahead, his cover and operation would be blown. He pulled strings in the background to get the trial abandoned, which the director of public prosecutions would have had to agree to if going ahead represented a threat to national security, as that would trump getting justice for a murder.
'The Crown' - i.e. the prosecution therefore withdrew, offering no further evidence, so the trial collapsed.