I was surprised when you said you were only getting 8Mbps against a potential of 14Mbps on O2 because my experience of O2 is that (on LLU) you get download speeds in line with sync at all times.
Your stats show that you are getting download speeds in line with sync because you are only syncing at 10Mbps and slightly north of 8Mbps is all you can expect after overheads.
The real question is why are you only syncing at ~ 10Mbps when you have 26.5dB attenuation - I get faster than you with 37dB attenuation.
You say that you have tried in the test socket - I'd suggest that it would be informative for you to do that again and post up the stats with it connected like that.
Do you have a burglar alarm that dials a monitoring service?
If you can't fix it with a hammer you've got an electrical problem.