If you as a professional aviator know this, why did the crew not?
I'd have thought the crew were well aware of the possibility of fog but their destination had CAT III runways with an RVR limit of 200m, with a
probability of 600m
at the worst on the forecast. The RVR appears to have dipped to 175m for a short period around their arrival time, coincident with Montevideo going below limits as a CAT I alternate (with a similarly benign forecast).
The BA245 has a block time of 13h40m, according to the timetable, so would have been pretty heavy for a full load down to EZE. Due to the sector length, to get a certain amount of extra fuel at destination would require loading considerably more out of London, possibly compromising the commercial payload.
Occasionally, despite the best planning, things don't quite go the way you'd like them to. All aviation proceeds on a balance of probabilities, so every-now-and-then, you get a five-sigma event or something like that. If 10% of flights were declaring emergencies and landing out of limits, then it would be raising eyebrows, certainly at the regulatory level. Once in several decades of daily operation is interesting but not really significant.
175m RVR on a CAT IIIa would be virtually indistinguishable from 200m, just that the limit is set at 200m. It's not like descending to 4,000' when the terrain is at 4,100' ahead of you. I'd hazard a guess that they probably had the required visual reference (which isn't much) at 50R, anyway. If the RVR had been 200m during the approach, then had deteriorated to 175m below 1,000',
they would legally have been able to land, given they got the above reference and there would have been no sensationalist discussion. The difference was that due to legacy rules there was an approach ban in force...