I've taken the train from Kings Cross to Edinburgh as well, and it is a very nice journey, especially as you get further north and see the coast. I went in February, and it was pretty cold of course, so you might be right about going in the summer when your parents visit then. Keep in mind, too, that Edinburgh is a popular New Years Eve spot, so if you were traveling then you may want to keep that in mind. I will probably be making the same trip again at the end of November. It is much nicer to go by train, and considering the hassle of the airports, security, etc., it is only a couple hours longer travel time, and a lot more relaxing.