Trolls are large in 5E, which is unhelpful, but they're listed as 9' tall in 2E. A 9' tall person with the low end of average BMI should weigh about 310 lbs. Depending on where you draw the line on what's edible, you could get as much as 80% of that weight in food. For the average troll that's 248 pounds of food. As long as you don't use acid or fire, it'll always grow back. The average troll has 84 hit points and regenerates 10 hp per round, or about 12% of itself every six seconds. Convert that to weight and it's 29.76 pounds every six-second round of regeneration, or 4.96 pounds every second. There are 86,400 seconds in a day...so there are 428,544 pounds of troll meat in a day...
London had a population of 5.567 million in 1891. Assuming the 5E rules of one pound of food per person per day, you could feed that population on 13 trolls a day.