I'm sorry but your sentence "it is true that, since the mid-1990s, the majority of young U.K. artists stopped copying the diction of their American idols - as had largely been the convention since the Beatles - and started singing as they spoke." is complete rubbish! UK artists copying mimicing the diction of Americans? Ha. Check out Green Day, little bit o' Jonny Rotten in there? In fact i'm struggling to think of any decent british acts who copied US diction explicitly..
Just another line in the americanocentric view of life.

Lily Allen's accent is put on by the way...she is in fact 'fraffly porsh'.

Sorry this turned out to be a bit of a rant...you do mention some good artists though (exclude Lily Allen...). Alex Kapranos has produced a fine album with the Cribs. Top notch. Foals? Hopefully they're better when not a support act, them supporting bloc party was very miserable indeedeo.