Just try to consider the logistics of what you are suggesting. Imagine, for example, LHR and LGW are fogged out while LTN and STN are in the clear. Are you seriously suggesting that all Heathrow and Gatwick flights should be rescheduled to go to Luton or Stansted instead? How do you suggest this would work air traffic wise or runway-wise? Where would you park all the additional aircraft? How would you find room for thousands of extra passengers? Where would you find the additional baggage handlers, check-in staff, gate staff?

This isn't just about YOUR flight. When an airport is weathered out it affects lots of flights, and there is no way one could simply divert all these flights to another airport. Hence the rule that if an airport is closed for weather reasons (or other reasons, such as an incident) then only those flights already airborne get diverted to another airport while those that are still on the ground stay on the ground.

I'm sure that if you think about it you will agree that this makes perfect sense!