You can compare flights between cities pairs using the CARI-6 program.
FAA Civil Aeromedical Institute - CARI
I researched three long haul city pairs between the U.S. to LHR vs. Tokyo. Radiation on the LHR flights were slightly higher even though the flights were shorter. With shorter flights a greater percentage of the flight is spent in the climb or descent vs. a longer flight. The shorter distance is flown, on average, at higher altitudes, so the net impact between the different city pairs is very small.