Re Heathrow Director
I presume that a new instruction as I never heard it before. Is there dispensation for a displaced threshold?
This side of the equator it's a runway separation standard. This is the Spectator's Balcony and I did point out that I wasn't being too technical.
The preceding aircraft must have vacated the runway and be taxiing away or, airborne and have passed the point at which the landing would be expected to vacate the runway, before the landing crosses the threshold. (Different for light aircraft.)
So if the landing is in the flare and crossed the threshold when the go around is given, chances are that the separation standard has already been broken.
A displaced threshold is just that, a displaced threshold. Mind you it gets a bit more complicated if it is dislaced for landing but not for takeoff.