After a certain population has been reached, all new citizens will be sad (angry) ones.
When sad faces outnumber happy ones, you get rioting.
You need to promote happiness. Luxuries make people happy The tried and true short-term fix is to create an entertainer (Click on the map in the city display to take a square out of production). Entertainers produce luxuries. If you can't afford to sacrifice the food, you can increase the luxury rate.
The long-term solution is to turn angry people into content ones. Build a temple. Followed by a colosseum and finally a cathedral when the city gets bigger.
You could also build settlers to keep London population down. It's good for the Empire to populate the world with more English cities.
Check the Civilopedia for more details on all this stuff.