You could fly out with easy jet from gatwick to sofia on the 30th for £85 per person, but getting back might be a problem until holiday season over, so stay on if you can.
BG school hols are until 10th Jan after that flight back maybe better.Best -Lock the flat for Xmas or drive to Bg with 4 people and back. -it takes 25-30 hrs non stop via Serbia one way.Still cheaper than flights.Mind you stop on the way in France,Swiss,or Austria and do it there if you are still hungry drive to the edge of Europe and have it in BG.