OK:
Imagine you have a flight from Gatwick to Paris. If the return flight departs to London and then Gatwick closes while the flight is underway, the flight diverts to an alternate airport (probably Luton or Stansted seeing as these are both easy bases). Right?
Yes
But if LGW closes just before the return flight departs, could you still depart anyway and change the destination, informing the passengers that they're now going to Luton/Stansted? I imagine this would benefit many passengers in that they'd arrive at another London airport rather than remaining stranded in Paris. Does this happen?
The flight-plan would require re-filing. It is unlikely the pax would be informed until near to landing. LGW might, after all, re-open en-route.
Also, if you are Gatwick-based easyJet crew and you divert to Luton/Stansted, do you have to make your own way home across London? Or does the company put you up in a hotel at the diversion airport because they are going to need you to re-position the aircraft back to Gatwick when it re-opens (i.e. today, Friday)?
Judgement call by company. Either:
Keep the crew there at the airport for a quick re-opening of LGW
Hotel the crew for a fresh start in the morning
Position crew (by company) to base and re-pos fresh crew/use local crew in the morning
Any of the three.
If you are really unlucky, pos the crew by road to Newcastle where you are short of a crew for a night Teneriffe the next day