What are the best options for a train travel between Edinburgh, Scotland, and Dover England?
go to London Kings Cross, frequent fast train service. Cross the road (side door to side door) to St Pancras station and get high speed train to Dover. Use www.nationalrail.co.uk to check exact schedules and fares, you can buy it as a through ticket or in two parts if it's cheaper. Takes about 6 hours.
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.