Hi,

You can calculate it from the demand and supply sides:

Demand side:

1. Population of London
2. % of the population that orders food on demand (use the sample of your friends and family)
3. Frequency - How many times per month do an average person order food on demand (Again I will use a personal example taking a sample of my friends and relatives. The main tip here - take the weighted average. Remember that only a small share of users is frequent (more than 4 orders per month) but the average user will probably order once in a month.
4. Deliveroo market share - ask the interview, or make an assumption (e.g. out of 10 couriers I see in the street 7 are Deliveroo)
5. Total Deliveroo orders per month = Population * % use on demand * Frequency
6. Assume the number of orders an average driver is making per month (20 working days * 8 hours per day * 3.5 orders per hour (taking into account that utilization is not 100% and that a driver is carrying more than one order))
7. Number of monthly active drivers = Total orders per month / monthly orders per driver

Supply side / sanity check:

1. When I open the Deliveroo app I see xx couries covering some area (at least it works for Uber)
2. Assume some utilization rate for peak / off-peak and apply it to get the total number of cars in your area
3. Multiply that area by the number of similar areas to cover the city (adjust to how busy your area is)

Best!