The biggest difference is that there isn't low level military helicopter traffic in the London Helilanes, mil traffic has to operate at the same levels and with the same clearances as civ helis (with minor exceptions using barracks LS's for example).
Similarly helo traffic crossing LHR has to comply with very strict clearances and routes - not at low level either.
So someone in the FAA signed off on having dual runway ops (IFR or VFR on one freq) with low level mil traffic (VFR only) on another freq. I would imagine the investigation will look closely at who approved that and what procedures are in place to mitigate the obvious risks.
It looks like airport efficiency (increase in traffic throughput) may have been prioritised over safety.