I don't watch much on the BBC but have to say I have never had any problems with hearing the dialogue on their shows. You may be right that it is sub-optimal but it's still not a struggle to hear it.
I did watch a film recently (the one where they go inside people's dreams, forget what it is called) and I did have to crank the volume on that to hear some of the dialogue, could well be the result of a mix-down to 2 channels as you say. I know that big Hollywood films spend a fortune on sound and use state of the art equipment so I doubt that the engineers are not up to the job, at least there anyway.
I don't recall what channel it was on but I do notice that there is a fair difference in level between some channels. Agree that old westerns and such the dialogue is very clear, but then they will be in mono for the most part.
What you are talking about does sound like so-called 'cocktail party effect.'
Maybe EQ it up in the 1-3Khz region, see how that goes?
Current Lash Up:
TEAC VRDS 701T > Sony TAE1000ESD > Krell KSA50S > JM Labs Focal Electra 926.