I've got mine booked on the same day - travelling down the night before!
I'm going to be wearing a full suit with tie, you can never go wrong interviewing in a full suit.
As for booking a return train, obviously the best thing to do would be to book an open return, but if you can't do that, I'd personally play it safe and book beyond 6pm - it's "expected" to finish at 4pm but I think leaving a 2+ hour window would be smart.
I really don't know how to prepare for the competence tests, all I've been doing to prepare is reading that 26-page document every day... still haven't memorised any of the aircraft info on the last few pages yet!