In a previous incarnation as a controller in this piece of airspace a very similar incident happened to me a few years ago. This also involved an airliner from across the pond arriving at Heathrow and a London City departure. The QNH was 992mb/hPa and when given this by the previous controller along with clearance to descend to 4000ft the pilot read back "2992 millibars" and descended with 29.92 inches set on the altimeters. Unfortunately this incorrect readback was missed. The resulting separation was around 500ft.
Not wishing to cast any aspersions, I offer this out of interest only.