Your theory has its ups and downs
my aunts exchange Aunts Exchange
was enabled in 2005 and is having 21cn wbc ready at the end of the year
whereas my exchange SCOL
was enabled in 2004 and has nothing new on its page!
truro,newquay,penzance,bodmin,st austell all 21cn exchanges and they were enabled in september 09, 8 years after first broadband intall so theory may be right but, it may be due to demand that they are rolling out 21cn in different areas first
Who knows with bt! (British Twits)