I figure BT took some stick for being late with a lot of exchanges, so they tended to give the "coming soon" exchanges a date within a certain quarter for 2012, but labelled all the "coming later" exchanges with a raw "2013" date.
I also figured that we'd start to see the 2013 dates crystallize into specific quarters as we got closer, and that seems to be happening: The Openreach "coming soon" list
now includes exchanges for "Mar-13", but nothing beyond.
Presumably this means all the future exchanges
are sometime in the following 3 quarters. Nothing listed for 2014 yet.