What are the best options for a train travel between Edinburgh, Scotland, and Dover England?
Sleeper from Edinburgh to London then train to Dover is another option
The options to an extent depend on the date- for instance most September weekends the East Coast main line is closed from Edinburgh to Newcastle for annual engineering work, with diversions via Carlisle adding 2 hours to the journey time (the direct replacement buses are as fast as the diverted train)- so an alternative daytime route like EDI- Euston (change at Carlisle or Lancaster) may then come into play.
It also depends whether (as I hope) you plan to arrive in Dover one or more days early, if this is for a cruise.