On this subject I use: https://www.lightpollutionmap.info/#zoom=9&lat=6724317&lon=-84999&layers=B0FFFFTFFFT
I have compared my measured SQM-L readings at places in Berks/Bucks and Oxon and find a reasonable good correlation with the map in the range 20.2 to 20.9. I use the LP map to spec out places to observe from. Ive not done an Orion count from the 20.8 locations, but i hope to do this.
British Astronomical Association
Supporting amateur astronomers since 1890