That makes no sense, I'm sure in the US we've had many more race riots and such (The Rodney King riots and the riots when MLK was assassinated come to mind), but we still offer freedom of speech to idiots.
Besides, from my cursory reading of the Brixton riots, the protestors were mad at police inaction, not hate speech.
Half of the Brixton riots were triggered by someone dying in police custody. The other half were triggered by the Met shooting someone. The Met is regularly accused of institutional racism, so you can see where I'm going there.
The fact there was no right to free speech in English law until 1998 may also be related. (",)
Besides, from my cursory reading of the Brixton riots, the protestors were mad at police inaction, not hate speech.