On your problem - do you know if you are on the 20CN or 21CN equipment at the exchange? Apparently the 21CN kit can help.
21CN exchanges have support for ADSL2+ which generally gives you an increase in throughput unless you've got really abysmal line stats. I was ~2Mbps on ADSL1 but went up to 3-4Mbps on ADSL2+. On FTTC I get ~27Mbps as I'm about 1km away from the cabinet. Its not quite the 40Mbps maximum that the current BT FTTC product gives you and its only just above what the maximum for ADSL2+ is but its still a very nice upgrade

I was with A&A at my parent's house and they are known to be good at getting the best from a line and if there are any problems, getting them fixed.
The only thing extra they do for you is allow you to adjust the stability and interleave settings for your line. Apart from those settings, there isn't anything they can do in terms of "getting the best from a line" though.
I agree that they're very good at getting a line fixed though. BT have a habit of rejecting or just mysteriously closing trouble tickets so you need your ISP to be vigilient about actually getting BT to do something about it.
I used A&A for ADSL and now for FTTC with baby jumbo frames and native IPv6. No complaints at all about their service.