Thank you, very interesting.
Where I am here near Cambridge and the observatory in La Palma were both rated Bortle 4 in 2015. However the former is near the top of the scale and has a sky brightness of 20.58 mag / arcsec^2 and the latter 21.58. I suspect the difference may be more marked now. Certainly the sky looks much darker in LP and the Milky Way is spectacular on a clear night at new moon. M31 is obvious from there but difficult from here in the UK. So many stars are visible that I have difficulty recognizing constellations!