I think the script has a few major flaws. like, as is, all life on Earth is 100% fucked. so, the story as is, should be about these people coping with the fact they are the last humans on Earth alive, and really are also going to go quietly.. into the night. and I couldn't ever really give much of a shit, knowing they are all dead anyway.
if they wanted to keep the action script as is, they should have established underground IS safe. so, lots of people will survive. But, THEY can't get to a bunker fast enough. Its not like there are just bunkers everywhere next to airports. so, need to stay ahead dawn, to stay out of the sunlight, to get to a bunker they know about. plants, and other animals would still be dead too, so still fucked in that situation as well..
or, just have the solar flare last 6 hours, so Europe is fucked, but not the entire world. and then just rewrite the script keeping that time frame in mind.