Ah yes Northolt is absolutely awash with fast jets arriving unanounced from all directions with fuel emergencies!
As for using 500ft separation in places other than Heathrow Zone - simple - because JSP whatever says it can be done.
However just because it says it can be done does not make it safe. If 500ft separation was safe in civil operations we would have double the capacity in the airways!
Similarly one has to question the safety of the altimeter setting procedures in current use.
Regards,
DFC