Hi Kris, I've enjoyed several train trips. The best experience I found was first class with Virgin, a complete breakfast on one train where others were just water and biscuit pack. I can't afford flying first, so it's a nice change to do first with a train company. Even with going first, on some trains it's good to get a reservation (they're free) when the time you're traveling is around commute time.
I did do the Edinburgh to London trip and saw the scenery, but it's only on the upper portion, still it's there and was nice. Going south, you want a seat on the left; going north, it'd be on the right. Taking a train trip once you're in Edinburgh so you can go over the Firth of Forth bridge is fun :-)
My experience with Virgin was Edinburgh down to Carlisle on a morning run, thus the breakfast pack. I would hope an afternoon/evening trip from London up to Edinburgh would be just as nice. I took a morning train from Edinburgh down to London, with a break to visit the railway museum in York, and the morning views of the ocean were nice, but no Virgin breakfast pack ... decisions have to be made :-)