I have now carried out a full and detailed study of the whole scones, jam, and cream situation. This is based on scientific experiments involving a great deal of personal sacrifice to determine the answer (i.e. eating a lot of scones).
And, the answer is:
It depends!
If you are using clotted cream, the cream is too thick to spread on jam, so the cream has to go down first.
If you are using whipped cream, it's impossible to spread jam on top of it, so the jam has to go down first.
Using butter is just not acceptable under any circumstances.