I am confused by the myriad train companies that seem to service this route. Can anyone recommend how to buy this ticket for my trip next year? (I won't need a rail pass, it's just a one-leg journey). Thank you!
Those folk have got it. East Coast or Virgin. The two routes don't overlap. Where have you seen "myriads"?
There are two routes you can take. The East Coast route which is faster to Edinburgh is operated by East Coast (http://www.eastcoast.co.uk/). It may occasionally be cheaper to buy a ticket from London to Edinburgh via the West Coast route (http://www.virgintrains.co.uk/) but it will take longer and you will usually have to change trains in Glasgow.
I booked my train ticket through http://www.eastcoast.co.uk/. It took about 5 hours from London to Edinburgh (Waverly Station). I know it sounds like a really long ride but it was really nice to relax and enjoy the beautiful scenery along the way. Enjoy your trip :)
The standard 2nd class fare is well over £100.00; but if you book ASAP on the East Coast site, you can get a discount fare as low as £17.00. You can book up to about 12 weeks in advance. Discount fare tickets are for a specific departure date and time and are non-exchangeable and non-refundable. Be sure you can commit before booking.