Not about Southern but Heathrow Express. Coming back to the UK at Christmas and needed to use the H Ex on the 27th. Went to their site to book a ticket only to find they are shut between 25th and 28th. Not a real big deal I suppose as I was able to get a seat with National Express instead. I generally avoid Heathrow like the plague if I can but it is still the major gateway for visitors flying in or out of London and will no doubt piss off a lot of tourists who will end up on the wanky Piccadilly line. Not a great advert for UK plc really.
They are building CrossRail though which should make things better (and cheaper).