550m is CAT 1 Min for many operators
The aircraft was I believe crewed by ex Saudi Military crews.
550m: is a fully legal, under many ops manuals for CAT 1 approach. It is the lowest RVR permitted, 600m being the regular Min RVR.
HUD: on the G550, it is possible to "fly" a HUD approach, and see the lights before a non HUD approach would see the lights.
It is normal procedure to input to the HUD, the runway elevation as well as the desired glideslope, this provides a secondary visual, as to the ideal approach path. The GS pointer being the primary guidance cue.
It is also possible that the crew had trained to CAT 2, in the simulator, and felt that they were qualified to make a CAT 2 approach, if their operations manual was so approved.
Training to CAT 2 is common place, in the recurrent world, even if non in operations manual as an approved procedure. It is trained to provide confidence in the systems, in he event of an emergency requirement.
Stansted ATC, would normally ask the crew, assuming the actual weather at the time of the approach was below 55m, that they were CAT 2 capable. Perhaps the word "capable" was misunderstood.
All approaches in UK below Cat 1 minima are reported by ATC to the relevant authority, who pass the approach conditions to the operators controlling authority.
I also assume that they were destined to land at Luton, and diverted to Stansted.
I also (trust) assume that fuel criticality was not a factor.
Glf