If the key reason for the blockage is traffic not being able to turn left onto Stondon Park because they cannot access the turn left lane on Honor Oak Park, then the light phase that allows traffic to turn everyway (out of HOP) should come first. Then, the turn left could happen (though I think this would require extra lights).

In fact, my back of an envelope sketch of the light phases suggests that the turn left phase may not be needed at all if this was done. A simpler lights cycle, ideally in conjunction with a SCOOT implementation, would speed all traffic flow including the bus routes.