What fascinates me is the whole idea of room acoustic being basically invisible to us - despite measuring differently, we don't really notice the effects of room acoustics in the rooms we aren't actively listening in. Your friend's voice in an auditorium, for example, sounds as recognizable there as it does in a small carpeted living room. We can hear the differences in spaces when we are called upon, of course, but we tend to filter the effects, whatever they are, out.
If British listeners prefer the downward tilted response, I wonder if it is simply because they are installing loudspeakers in more reverberant rooms, or if as a culture they are more used to reverberant rooms, and prefer a certain kind of response? It is impossible to say how the every day content of our auditory experience affects that sense - I think you wrote on this website about hearing the Bavarian forest and understanding Beethoven in some new way - I found this fascinating. Who knows what the sound-environment was for him, or any of our composers, conductors, musicians, critics. There must be some internal reference for a composer - some amount of reverb and space that they take into account when they are imagining the interactions of instruments with different timbres - perhaps different compositions should only be performed in the hall that the composer knew best when he developed his sense of instrumentation?
The question raised by this lecture, of course, is how our perception of sound has changed with 'advances' in architectural acoustics. It seems like the historical trend has been towards quiet, but we all know that architectural acoustic panels work best on the higher frequencies and not very well at all on the lower ones. This, combined with a trend towards spending lots of time indoors, represents a change in our external acoustic environment, which must I feel correspond to an internal change of some sort in perception. I can only imagine what listening to headphones every time you go outside must do.