-
Two braking mechanisms is a legal requirement for UK roads so I don't see them being able to ignore it.
hippy-
Yes, while it doesn't matter on the uphill closed road course itself, the race HQ/changing facility was in a nearby school a few years ago (or some other public building, I can't remember) and probably will be again, the run back to the start is down Highgate West Hill, which won't be reserved for the race (they probably wouldn't take people's promises to walk down at face value), and people would most likely warm up in the surrounding streets, so there's plenty of need for a brake.
It's not a legal requirement while you stay off the open public highway (they've run brakless crits here on closed roads), but the organiser obviously has discretion to demand it if there's a good chance people would be riding around, which there is here. Regardless of discretion, it's also probably an insurance requirement.
Oliver Schick
Tempting. Have to laugh at the front brake being needed haha