Every course has a finish line. Not every London course uses the official start/finish banner. the classique starts and ends with the sprint banner much like the flat course on Richmond.
Don’t forget your mileage for the ride will be higher. You may ride 0.5 kilometers before starting a 15.5 km course. thus, your total ride will be 16 km before you cross the line. And the flat courses are about 4 or 5 km from when you start riding. It has thrown many a racer off.
Check the course on Strava first, esp race courses on zwift. The laps on volcano circuit often confuse people the first time.