ETC
In a case where the pilot deviates without asking, should it result in less than standard separation, it would be a "pilot deviation," and not a controller error/airprox here in the US. It is certainly not unusual to put aircraft on headings when in close proximity. BTW does ICAO have a "degree divergence" rule where you can use less than 3 miles if aircraft are assigned diverging headings by ??? degrees? If so that may explain why London TMA puts so many aircraft on headings. It's only allowed in terminals in the US last I knew, but center/area has been trying to get it in areas where 3 miles separation is available.