I consider that it is prudent to keep a good stock of food, fuel, and other supplies in case of any untoward events.

Not specifically for Brexit though.
Any significant shortages of imported would surprise me, Why would the EU obstruct or delay food exports from France* into the UK. They will have to deal with some very angry French* farmers if the export earnings of said farmers are affected.

Still well to be prepared though, just in case.

I consider it more likely that exports FROM the UK and into or via France* will be interrupted by some obstructive French* officials who may be under instructions to look very hard for any errors or non compliance with the rules.

I consider street protests to be possible, but not primarily over Brexit, even if that be the catalyst.
Violent protests over general dissatisfaction with the government seem more likely, as have recently broken out in France.

*there is of course more to the EU than France, but France IS a leading EU power, and is the nearest part of the EU to the mainland UK.
Most trade between the UK and the EU is either with France, or goes via France.