That’s almost impossible to answer. Flights can vary wildly depending on: number of stops, airline airport flown out of, time of year, day of week, time of day, special events in either location, etc. You may also want to factor in something completely random as well, say the color of the tie that the BBC reporter is wearing on the evening news.

Hong Kong to London and back should have plenty of competition on the route, which helps keep prices under control. It also should have plenty of business travelers, which should help keep it expensive.

Your best bet is to find your favorite flight aggregator and do the heavy lifting yourself. Pricing flights is a black art.