Jam and cream is the cream tea taken off the scone and rebuilt as a sandwich, and in its fullest form it is an indulgent doorstep: thick soft white bread, a generous bed of clotted cream, a heavy spread of strawberry jam, closed and cut into substantial halves rather than dainty fingers. The defining move is that it transplants the scone's filling onto bread without scaling it down. The cream is laid on thick because clotted cream is the whole pleasure of a cream tea and a thin smear would be a different, meaner thing; the jam is generous on top of it. This is not a restrained sandwich and is not pretending to be. It is the cream tea argued at full volume, the bread standing in for the scone purely because bread is what was in the house.
The craft is layer order and holding a rich, wet, unstable filling inside the bread. The cream goes against the bread first and the jam on top of the cream, the same order argued over a scone, because clotted cream is fat enough to seal the crumb against the jam's moisture where the reverse leaves a sodden slice. Clotted cream is dense rather than slack, which is what lets it carry this much jam without sliding out the sides the way whipped cream would under the same load. The bread is thick, soft, and plain on purpose: a doorstep cut takes a generous filling without the bread vanishing under it, and a chewy crust would be the only thing in a soft sweet sandwich resisting the bite. It is built close to when it is eaten, because cream and jam against bread do not keep.
The variations move the cream or the scale rather than the idea. Whipped double cream in place of clotted gives a lighter, less indulgent build that needs eating faster; a tarter fruit jam cuts the richness for those who find clotted cream too round; trimmed thin and crustless it becomes the afternoon-tea version rather than the doorstep one. Each deserves its own article rather than being crowded in here.