I remember when backpacking around the Southern Hemisphere a while back, I arrived at Christchurch airport in NZ at 1am due to a coach delay.
Had no option but to stay in the airport.When I got to departures it was closed.
I spoke to one of the staff who indicated that they kept just the arrival section open for people to sleep if waiting for a early departure or arrived late.
Security would check that they were actually travelling through the airport and not just dossing!
Don't know the layout of SEN or if it would be possible but I feel also that they should at least have a duty of care .The girls would have expected that all London airports are 24 hr and to be honest that's pretty fair!