This is all very hypothetical, but I think it's worth thinking about given how much Robinson is hyping this up as a possibility. There are also lessons we can learn from what happened when Lee Rigby was killed in 2013.
When Rigby was killed there were flash protests in the area where the killing happened, mass protests in central London with thousands of attendees, attacks on dozens of Mosques across the country and the far-right in the immediate area where the killing happened was strengthened significantly. Protests took place in the immediate area for several years before petering out although they can still happen occasionally.
If Robinson is killed in prison (which is highly unlikely), I would expect the reaction from his supporters to be significantly worse than it was when Rigby was killed. If his assailants were Muslim (or if people were told that they were Muslim, the truth doesn't really matter to his supporters), I would expect more Mosques to be attacked and potentially even some pogroms to happen in some parts of the country. Organising against that could be incredibly difficult as it'd basically be inter-communal violence.
The mass protests in central London would be a lot easier to respond to, if the left was willing, as it's just about putting numbers onto the streets. Although Robinson could easily get crowds of 20,000+ in death who would be more interested in rioting than they were when he was jailed. There is also the likelihood of more lone wolf style attacks but there's very little which can be done to stop those.
Robinson has just been sent back to prison for 10 weeks.