The operator has its own operating minima which appears in the Ops Manual, a compilation that has been agreed upon by the Operator and its relevant CAA.
However, the minima that appears in the ops Manual is unlikely to be lower than any minima stipulated by the relevant CAA and nor will it be lower than any minima specified for a specific airfield by that relevant CAA.
Now consider this if will please.
There you were at Heathrow, all loaded up with passengers and fuel for Nairobi, ready to blast off into the unknown perils of the African sky. Let its suppose that on this specific day Heathrow had an RVR of 400mtrs but was forecast to have a visibility of less than 200mtrs for the period of an hour after your scheduled take off. So without special equipment over and above that of a Cat 1 ILS standard, you can’t return to Heathrow in the event of an engine failure after take off. Instead of being able to Ireland at Heathrow, you need an alternate for departure. Now, let us assume that Gatwick and Stansted are both enjoying a sunny day, so you could theoretically use either one of these as an alternate for departure after a take off from Heathrow. But perhaps KQ doesn’t want the engineering costs involved in landing a sick aircraft as far away from its maintenance base as LHR? Perhaps the weather minima for take off from LHR in KQ’s Ops Manual has to be such that the aircraft could return Heathrow? But if Heathrow were closed to all but Cat 111 operations and your aircraft is equipped with Cat 111 then what could be the problem? We’ll, in continued hypothetical vein and perhaps because of airline economies, suppose KQ hasn’t stumped up for the training and recertification of operating crew to Cat 111 operational standards? So, although Heathrow has a Cat 111 runway and the aircraft has auto land, the crew can’t operate back into Heathrow in the event of a safety requirement to do so. Operational considerations could quite easily prohibit the crew from departing from Heathrow when the aircraft cannot return to that airfield of departure. Thus the crew could have been operating entirely legally or reasonably, or even both ways, in delaying the take off.
Suppose too that while all this has been going on the crew duty clock has been ticking as well. They’re running out of time to complete the flight. Now you might be faced with a situation where the crew would knowingly be departing an airfield and flying into discretion or exceeding duty time. There’s no standby crew in London of course? Airline economics again perhaps?
Thus, although weather conditions might have dramatically improved and Heathrow was now wide open, the crew couldn’t legally depart the field.
That above load of not so theoretical waffle all sounds damningly complicated but it can be the reality in flying. Most things to do with aircraft are acts of God. It’s a miracle many of the crews out there get them to fly anyway. That’s why Airbus is so predicated on automatic flight. I suspect you’re lucky to be alive and should send €600 to the KQ widow’s and orphan’s pension fund at once.