When I lived in Derbyshire, I put jam on first, whether it was fruit or plain scone. Now I'm in Cornwall, it's always clotted cream.
Biggest advantage of jam first, is that you can spread a good layer of jam over the whole half and then spread the cream over the top. A dollop of jam on top of the cream, stays as a dollop. Very difficult to spread it around. Jam and cream must reach the edge for me.
My favourite is scone - jam - clotted cream - halved strawberry.
Of course, each half of the scone has to be treated the same. Never make a scone sandwich 