Certified in 1992 means designed several years before that, which is just about pre-GPS.
With INS with DME/DME corrections, GPS was never a priority. Only the most modern airliners have GPS integrated into the INS, and
AFAIK they have three of them.
You are living in another age
There are numerous ways to end up with crap GPS reception. VHF interference (11th and 13th sub-harmonic of 1575MHz) is a common one (try transmitting for 20-30 secs on 121.2 and see what happens to the GPS satellite signal levels). ELT pi-tank resonance (triggered by VHF transmission anywhere around 121.xx to 122.xx) is another. Crap installations are another
So if the RVR is greater than 550m, a crew could fly a CAT I, II, IIIA or IIIB approach
I think there is a "story" behind this, too. In UK airspace, a single pilot is limited to
800m for takeoff and landing. With high intensity runway lighting and with an autopilot which can fly the ILS all the way down to the published minima (say 200ft DH) this improves to
550m which is the normal published vis figure on Jepp plates.
That's AIUI, anyway.
Incidentally, I am told the standard answer on an IR test is supposed to be 800m and if you answer with 550m then you fail, because you are not supposed to use the AP.