You should look up the checkered history of the Concorde flight from SIN-LHR where the plane was painted in BA colours on one side and in SQ colours on the other side.

The plane was force of drop to sub-sonic speeds over some many areas, it really did not work. Supersonic speeds over coastal waters and populated area was not allowed by certain countries for fear of sonic booms. I not sure flying supersonic over the Med was allowed - fishing issues.

TALT route was one of the few wokable ones since it was not over populated land or fishing waters.