Riots require young (under-40) fit people who live in cities.
The demographics of Leave pretty much completely excludes that category of people. It’s largely men over 50 (particularly over 55), and women who are even older, who live in rural areas or towns rather than cities. In order to even riot properly they’d need to journey to the bigger cities, and then do so. Which given the pathetic turn-out for pro-Brexit stuff even before the referendum, seems incredibly unlikely. And it’s not like the police are going to have many issues containing them, given they managed to contain people far younger, fitter, and more determined. Most of them would get out of breath from throwing a couple of bricks and have to head down to Weatherspoons for a calming pint and to take their heart medicine.
You might get the EDL/BNP types rioting, but again they’re not exactly numerous, and they fulfill every negative stereotype re: Leave, being explicitly racist bigots (usually with bonus homophobia, misogyny, and so on).
The big problem right now is that May is basically doing Project Fear x1000 “Now causing genuine large-scale economic harm to the UK!”, in order to try and arm-twist her stupid, pathetic back-bench bitchboys into supporting her pretty shit deal, because she is clinically obsessed with her “place in history”, and thus unless they take her deal (which is not by anyone’s standards, nor a good compromise - it’s sole merit is “it’s not No Deal!”), and lick her boots, so she can leave and say “I got Britain a good deal, now I’m retiring!”, and do a Cameron, she’s going burn this motherfucker down. That’s why she refuses to rule out No Deal, even though there is no sane reason not to rule it out, and it’s already costing us money and jobs, and will cost more every day.
And we’re stuck with a Labour leader who is thinking “Burning this motherfucker down might be a good starting point for The Revolution™… Maybe I’ll just go along with this…” and is also too polite and press-averse to go around bad-mouthing May, even though it would be trivial to wreck her shit by just calling her a lunatic and pointing out that she’s actively trying to damage the country.