I've added a turn restriction: https://www.openstreetmap.org/changeset/146197367#map=19/51.56843/-0.12311&layers=N https://www.openstreetmap.org/relation/17019033#map=18/51.56904/-0.12363&layers=N
There is no restriction when approaching northbound on Hornsey Rd. The only sign is a warning about restricted width of you turn left (traffic calming measure further down Fairbridge Road).
One thing I'm not sure about is whether cars are allowed to approach from Spears Road and take the staggered route "straight" through onto Fairbridge Road. I'll go back and check some time soon (very near me)