Which rail service is best?
That is a long rail journey - as Philip notes it is so long that they run a sleeper service on the route. I used to regularly go to Inverness from near London, and on that basis I ask "have you considered flying?". The low cost airlines do run London - Inverness.
I took the train from King's Cross to in ness. It left sometime between noon and 1 pm. I got in around 7 I believe. There was free WiFi all the way. I actually enjoyed the journey. I had flown over from Wisconsin the night before. I took the tube to King's Cross. Paid an expensive fee to leave luggage at the left luggage department. Had breakfast. Saw a wonderful exhibit on Henry VIII at the Library before catching my train. Pam