I don't know why you'd want to add ED to Steam. ED has it's own launcher which keeps the game up-to-date, so no need for Steam.
Horizons is also on sale for £19.99 --- and they seem to have renamed it "season pass" which is more in line with the easily-understood terms used today. From what I can tell, you can buy Elite on sale at £10, then the "season pass" called "Horizons" for £19.99, or you can buy the bundle at £39.99. Makes more sense now.
However, doesn't a "season pass" mean you get EVERYTHING for a game? AFAIK, games like The Witcher 3 only have one "season." I guess ED will have a few "seasons," if that makes any sense.