When I was looking at the start of the year, I think another option was Xilo.net - looking on their site the package seems to be in their Pro range and the Unmetered one. They also have lower usage tiers.
Ended up going with PlusNet though as I had been with them before and generally the opinion seems PN is pretty good now. Annex M is only available on PN Business packages, not residential, so does mean a more expensive package but also gives you access to the Business support which is free phone (think residential is too) and has far shorter wait times. Suggestion is the business support team are also more experienced staff having been at PN longer too.
I think AA did an Annex M package but for my needs it was more expensive than I really want to pay (compared with PN which is unlimited and working as well as Zen or BE ever have)
plusnet ADSL2+ Annex M (and previously Be* Jan 2009-Jan 2014, ZeN Dec 2006-Jan 2009, plusnet Aug 2004-Dec 2006, ZeN Apr 2003-Aug 2004)