It's just what BACF do. Trying to utilise an aircraft Sunday morning rather than it being sat on the ground at EDI.
A nice little round trip to Gatwick, before a lunch time return to City airport, landing no doubt shortly after it reopens.
Also, a short 3 sector day to get that crew back to home base rather than having to fly them back on a BACF flight or via LHR and a long taxi.....