Just wanted to add an update to this thread:
I exchanged emails with <name removed by MOD>, General Manager SME Marketing at BT Business, and the reason BT Business/Retail do not offer this product is problems with predicting the upload speed on any given line at the point of sale. <name removed by MOD> said the last time they reviewed this product the BT Wholesale Line Checker results for Annex M were not reliable enough therefore they would not offer it. However she did say "Should the predictability be [improved], we will reconsider launching the product".
BT Wholesale are pushing this product to all ISP's as a replacement for SDSL, particulaly where Fibre is not an option, so it seems the ball is in their court to convince BT Business/Retail that the predictability is good, and that the product is worth offering.
It's worth noting that Plus Net (a wholly owned BT subsidiary) offers Annex M where available on all it's business broadband products for an extra £10 per month.
As there are many providers who offer Annex M service I don't believe BT Business' argument of reliabile line speed prediction problems, more likely that they don't offer the option of Annex M as it's not mainstream enough to make sufficient revenue. When my broadband is out of term, I will be switching to Plus Net.
