This recent news story resulted from the Captain's poor choice of words in his announcement to his passengers. Usually when a flight is unable to land in poor weather, it is because the ceiling or visibility is below the required minimum value. This value varies according to many factors - pilot training and experience being just two of them. Others include the approach facilities installed in the aircraft, the runway electronic approach facilities (instrument landing system) in use, runway lighting system in use, wind conditions, runway conditions (ice or snow), even daytime versus nighttime in some cases.
The more "diplomatic" way for a Captain to announce that he is unable to land due to poor weather might be: "The weather in Paris has deteriorated below the required limit. We are diverting to our alternate airport, which in this case means we'll return to Cardiff."
Obviously when he left for Paris, the pilot figured he'd have the needed landing conditions. But hey, weather doesn't always "obey" the forecast. This pilot's use of tech-speak and jargon was unfortunate and caused needless concern to his passengers.
Interestingly, this BBC news report used more comprehensive language in their headline:
Rules stopped pilot's fog landing