"They Said in Effect: Political Violence Is a Line We Won't Cross"

"After Olympia, Rothermere and the Daily Mail also withdrew open support and the rest of the British press effectively boycotted Mosley, reporting only the continued violence that broke out at subsequent rallies but not details of his speeches. Newspapers that had previously been sympathetic—often catering to a public taste for appeasement of Hitler—changed their tune. British Blackshirts were branded as thugs.
"That also made attending Mosley’s meetings much less comfortable for MPs and government ministers. It was only after this shift in the way fascists were treated in the press and in political discourse that the realities of the Nazi threat finally penetrated the British public, helping them to coalesce behind the war when it broke out in 1939."

Katherine Pickering Antonova at The Washington Post explains "[h]ow the British defeated homegrown fascism."