There's a fair amount of effort involved to support something like Annex M. For example you'd need to modify the existing Automated ordering/regrading process to support it, make product set price/feature decisions, online order process (with checks that Annex M is actually available on that number of course), training for customer facing teams, managed trial phase etc. These things will all take time to do.
That's not to say its not worth doing of course, BT's FTTP/FTTC products will take a long time to become widely available.
However Annex M is by no means available on every BT exchange supporting ADSL 2+ / WBC - so there would have to be some analysis done to determine the likely takeup/availability verses the effort to develop the product.
(you can see if a number has Annex M supported by BT wholesale using the availability checker at
http://www.btwholesale.com/adslchecker - if Annex M is mentioned in the results text then it is supported, no mention means its not)
Hope that helps!