Annex M pricing
Faster upload speed than uncapped ADSL2.
[quote=http://en.wikipedia.org/wiki/ITU_G.992.5_Annex_M]Annex M is an optional specification in ITU-T recommendations G.992.3 (ADSL2) and G.992.5 (ADSL2+), also referred to as ADSL2 M and ADSL2+ M. This specification extends the capability of commonly deployed Annex A by more than doubling the number of upstream bits. The data rates can be as high as 12 or 24 Mbit/s downstream and 3 Mbit/s upstream depending on the distance from the DSLAM to the customer's premises.
This is an easier to understand version - Annex M –What’s it all about ?
This shows the UPLOAD speeds available on various Plusnet connection types for BUSINESS products - Plusnet : upload speeds
[quote=http://en.wikipedia.org/wiki/ITU_G.992.5_Annex_M]Annex M is an optional specification in ITU-T recommendations G.992.3 (ADSL2) and G.992.5 (ADSL2+), also referred to as ADSL2 M and ADSL2+ M. This specification extends the capability of commonly deployed Annex A by more than doubling the number of upstream bits. The data rates can be as high as 12 or 24 Mbit/s downstream and 3 Mbit/s upstream depending on the distance from the DSLAM to the customer's premises.
This is an easier to understand version - Annex M –What’s it all about ?
This shows the UPLOAD speeds available on various Plusnet connection types for BUSINESS products - Plusnet : upload speeds
If you've got a good enough line it gives you extra upload, in theory up to 2.5mb - but the usual factors come into play.
EDIT - beaten by purleigh's much more useful answer
EDIT - beaten by purleigh's much more useful answer
Quote from: ianfretwell It'd be nice to have an official answer from PlusNet though wouldn't you all agree ?
What do you expect Plusnet to say, because Plusnet are resellers of BT broadband connections, and as with all the available broadband add-ons they pretty much just pass on the cost that BT charge them. These prices are about the same with whatever ISP you choose.
Your question should perhaps be rephrased as -
"how are BT allowed to get way with charging so much for minor technical enhancements".
or
"why do BT restrict the full potential our broadband connections and then charge considerable extra money for removing the restrictions"
Don't get me started on "Market 1" pricing in rural areas ! 
I do agree that pricing would be fairer if the amount BT charged was correlated to the speed that the customer actually received.
That would be a great incentive for BT to upgrade the rural exchanges to something better than "up to 8Mb" 20CN connection
However, as the original question of this topic is really about why are the small improvements made possible with "Annex M" or "Max Premium" so expensive compared to the price of the standard service.
For example you might have Plusnet "Essentials Broadband & phone" at £5.99 per month (assuming Market 2/3 and on 21CN ADSL2), but to add "Annex M" to provide an not guaranteed "up to 2.5Mb upload" improvement costs an ADDITIONAL £10, which is a grossly disproportional price compared to the basic product.
Although it could be worse, being charged more money for less performance on the SAME broadband product -
e.g. Plusnet "Essentials Broadband WITHOUT phone" at £15.49 (on "Market 1" 20CN ADSL), then add £7.83 for "Max Premium" for the not guaranteed "up to 0.8Mb upload" speed.
Suddenly 21CN with "Annex M" doesn't seem so bad
"Market 2/3" 21CN ADSL2 (2.5Mb upload/18Mb download) with phone at £15.99 for 10GB per month.
compared to
"Market 1" 20CN ADSL1 (0.8Mb upload/7.15Mb download) without phone at £23.32 for 10GB per month.
However both these cases are still very expensive compared to the basic "Essentials" product at £5.99 per month.
jim:green factual errors in post corrected at the request of the poster mod:end
I do agree that pricing would be fairer if the amount BT charged was correlated to the speed that the customer actually received.
That would be a great incentive for BT to upgrade the rural exchanges to something better than "up to 8Mb" 20CN connection
However, as the original question of this topic is really about why are the small improvements made possible with "Annex M" or "Max Premium" so expensive compared to the price of the standard service.
For example you might have Plusnet "Essentials Broadband & phone" at £5.99 per month (assuming Market 2/3 and on 21CN ADSL2), but to add "Annex M" to provide an not guaranteed "up to 2.5Mb upload" improvement costs an ADDITIONAL £10, which is a grossly disproportional price compared to the basic product.
Although it could be worse, being charged more money for less performance on the SAME broadband product -
e.g. Plusnet "Essentials Broadband WITHOUT phone" at £15.49 (on "Market 1" 20CN ADSL), then add £7.83 for "Max Premium" for the not guaranteed "up to 0.8Mb upload" speed.
Suddenly 21CN with "Annex M" doesn't seem so bad
"Market 2/3" 21CN ADSL2 (2.5Mb upload/18Mb download) with phone at £15.99 for 10GB per month.
compared to
"Market 1" 20CN ADSL1 (0.8Mb upload/7.15Mb download) without phone at £23.32 for 10GB per month.
However both these cases are still very expensive compared to the basic "Essentials" product at £5.99 per month.
jim:green factual errors in post corrected at the request of the poster mod:end
What hasn't been highlighted in this topic is that Annex M ALWAYS reduces the download speed.
Cheers Jaggies