She said Mr. Osborne then dived into the right-wing media sphere and “seemed brainwashed” by his immersion in that world. Devices seized by the police showed internet searches for a variety of such sites, including that of the English Defence League, a far-right anti-Muslim group.
Mr. Osborne was also found to have received an automated direct message on Twitter from Jayda Fransen, deputy leader of Britain First, another far-right organization. Some of the group’s videos were re-tweeted by President Trump last year in an episode that caused outrage in Britain.
During the trial, the court heard that Mr. Osborne, who is from Cardiff, in Wales, drove the van into the late-night crowd at Finsbury Park, leading to the death of Makram Ali, 51.
The area had been busy with worshipers attending Ramadan prayers, and though Mr. Ali had collapsed just before the attack, several of those who went to his aid said he had been alive and conscious before being struck by the van.
Mr. Osborne denied that he had been the driver of the van, saying it was driven by “a guy called Dave” who had disappeared from the scene.
But he admitted to an earlier plan to attack a pro-Palestinian march in London, which he and two accomplices believed Mr. Corbyn would attend.
Asked in court whether he hoped to kill Mr. Corbyn, Mr. Osborne replied, “Oh, yeah,” adding, “It would be one less terrorist off our streets.”
He added: “If Sadiq Khan had been there, it would have been even better. It would have been like winning the lottery.”
The Finsbury Park attack came after several other terrorist assaults in England. A month earlier, a bomb exploded at an Ariana Grande concert at the Manchester Arena, killing 22 people, and the man identified as the bomber, Salman Abedi, had links to the Islamic State. In June, attacks on London Bridge and at Borough Market, also inspired by the Islamic State, killed eight people.
There have been other prominent examples of far-right violence, most notably the 2016 killing of the lawmaker Jo Cox by Thomas Mair, a loner with ties to extremist groups.