This is silly. Not sure if its ignorance or what, but its not true to say any of this.
We have vast swathes of Far Right idiots. Who often "march" and it often descends in to violence. The EDL, BNP and Britain First and so on are one thing but then you have the actual full on neo ****s like the National Action. whose "marches" (where they're instructed to wear all black, cover their faces and bring concealed weapons) are only stopped by counter protests by anti-fascist organisations arranging for local community groups to counter them.
Left wing marches are largely peaceful affairs, having been on 5-6 in the last year without a single incident, often police involvement and poor tactical responses (like "Kettle-ing" on huge groups and murdering old men walking home from the pub) were what caused the tensions to spill out, since the police have stopped doing silly things like that, there hasn't been anything violence.