The Tomorrow Project by H Critchlow

Marianne, Downing Street press secretary, doesn't realise the futility until it is far, far too late. When the truth hits her, Marianne is forced to choose: stay with her family, or do whatever she can to help the doomed survive. As London falls, seven-year-old Maia is one of the last to escape the city.

In an evacuation camp, she binds herself to Finn: in the absence of everything she knew, he becomes her everything. Yet as the years roll on and hope fades, Maia sees the bubble of safety is also a prison. She realises there is only one choice: to leave the camp and find what remains on the outside.