Pc Ben Hannam, 22, had also pleaded guilty to possession of a prohibited photograph of a child.
Benjamin Hannam, 21, allegedly belonged to proscribed organisation National Action.
In an extract from his new book, Hope Not Hate head of intelligence Matthew Collins tells the story of meeting the National Action activist who blew the whistle on a plot to kill Labour MP Rosie Cooper.
Alice Cutter entered the under the name “Buchenwald Princess”, the jury was told.
They named their baby son after Hitler.
"The ideology, neo-Nazism, those seeking violent extremism – that’s still prevalent, and we have to remain vigilant."
Adam Thomas and Claudia Patatas were convicted at Birmingham Crown Court.
Lythgoe was arrested by police investigating a plot to kill Labour MP.
'Are you out to insult the public by not taking this seriously?'