-
Yes, that’s fine when it’s just one aircraft wanting to divert. The problem when Gatwick or Heathrow shut suddenly is that lots of aircraft at once want to fly to their alternate, which may or may not be able to accept them. They all have to find a diversion airfield that can accept them, and then be individually coordinated and re-routed by ATC, which takes time and fuel that you weren’t planning to burn.
Shouldn't all flights have enough fuel for 1st reserve airport though?