I always do scone, butter, jam then cream last. The jam is denser than the cream and can slide off the jam if you put the cream in the middle.
I also think its kind of assembled like a trifle, cake (scone) sherry (butter - yeah this one is a bit of a stretch) Jelly (jam - same colour and fruit based) cream (clotted cream preferably for scones, but I can go with whipped, as long as its not from a can) I know there isn't much logic to this.