Did you do all the agreeing by phone?
If so then it will have been recorded,so if you can get high enough up the complaints ladder you should be able either to get moved to LLU or be allowed to leave without penalty, as the phone recording will show they are in breach of contract.
If you can get to that stage, I advise to to get a MAC and leave. They might tell you you can't but I don't think they could force you to stay at this stage.
Look at this as well. There are several far better LLU ISPs and LLU resellers on that exchange, that you could ask about in "Which ISP?" Only problem may be if you want the cheapest possible deal!
Having said all that, you definitely still have some sort of line problem. I would expect a much higher connection speed than your current 6016kbps. Somewhere between 6500 and 7500kbps. Whatever it is is unlikely to be solved by a move to LLU, where you could expect over 8Mbps and perhaps approaching 11Mbps if the line was right There is no obvious IPStream congestion at that exchange.
I suggest you try connecting with an ordinary filter through the tests socket, on the wall at the back in this pic, and see what the stats are from there. There won't be any immediate increase in download speed even if the stats are better, so don't bother with speed tests.
Do you use a plug-in extension to connect the router to the faceplate, or just the short ADSL cable from the router? If you use a plug-in extension that could very well be the problem, so try without it even if that is inconvenient. Have you got a phone-connected burglar alarm? Have you got a Sky TV box?
I agree that a formal letter will be the best way forward to the head of customer services, explaining the situation and inviting them to listen to the conversation. I also have a telephone line with them that is also tied to the 18 month contract. I will see what their response is.
Regarding the line issue, I have never connected at more than 7200k I think was the highest when I was on O2 LLU about a year ago. I had no problems with them and regret leaving to be honest. I never got less than 6500k download speeds even during the evening peak. So I am surprised to hear that you think with my line attenuation I could be getting up to 11MB in perfect conditions. I am not complaining however!
The NTE5 Master socket where the line comes in from the street is in the front bedroom, then the secondary wiring comes from there to downstairs where there is a Sky box and normal telephone/answer machine plugged into a secondary wall socket. On my old setup I had a normal NTE5 faceplate in the front bedroom and the the modem plugged into the faceplate of that using a micro filter.
One of the Orange tech support guys suggested that my downstairs setup & sky box was causing the problems, as I had a microfilter plugged into the wall socket downstairs and from that a splitter on the voice side to split the telephone & sky box. He advised me to change it so I have the splitter plugged into the wall socket then 2 micro filters plugged into that (one for the sky box & one for the telephone). I did this, but no improvement, still roughly 6000k connection speed but between 500k and 5500k download speeds.
With my new setup, I have used the ADSL faceplate that I had spare that the BT engineer gave me and plugged the modem into that using the short grey rj45 cable. The filtered extension wiring comes from there straight to the downstairs wall socket where the splitter for the telephone & sky box are with no micro filter as I understand it is not required with the ADSL faceplate on the master socket. Result: Speeds the same as the previous setup.
I have done what you suggested and removed the ADSL faceplate and plugged the micro filter straight into the master test socket upstairs. I am using what I believe are high quality BT branded micro filters.
Here are the new line stats when plugged into the test socket (Netgear DGN1000):
Connection Speed/6240 Kbps/448 Kbps
Line Attenuation/39.7 dB/23.0 dB
Noise Margin/6.5 dB/20.0 dB
I have a spare Linksys/Cisco WAG120N router that I have just tried and that gets a downspeed connection speed of 6210Kbps down & 448Kbps up when plugged into the master test socket. But it does not seem to give any other line stats so seems at bit useless. I have now gone back to the Orange supplied Netgear DGN1000 as it gives slightly more info.
As you can see it is still 1000k less than what I was getting with O2 last year, could the problem be elsewhere?