Quite simple, all the fuel planning in the world goes out the window when the UK or large region goes to chaos and airports fill up. I've had more diversions where planned alternates have been denied, than accepted. Use common sense, and take what you need to be sure of the options needed in such situations. Perfect day, then fine, save the fuel, but your lovely little plan of filed alternate is useless when everyone else fills it up your list of alternates. Couldn't care less about pay when it comes to fuel.