AFAIK the router you are probably using is giving you a figure for the SNR Margin not the actual SNR, therefore there would appear to be possible issues with your connection when the exchange equipment has raised the noise margin to around 12dB which would also lead to a lower sync rate and throughput.
I'd looked at Kitz but my combined modem router does not support command line instructions so its not on their list of compatibles. As for the master socket not sure whether its an NTE5 or an ADSL Service Specific Faceplate (SSFP). As the house slave sockets don't require external filters it suggests the latter. As I remember there is a high spec twisted pair cable wired to an internal RJ11 running for about 3 metres to the modem RJ11. Its a bit of a hassle getting to the SSFP and using the test socket so that's a no go for the moment. The router stats do specifically refer to 'SNR Margin'.
I Did the BT Wholesale test as Plusnet recommend and it came back with the same speed of about 2Mbps.
Logging in to the portal as you suggested it says the speed it currently 2.1 Mbps.
The router ADSL stats is reporting a downstream speed of 2.414 Mbps. SNR Margin is 8.9 dB Attenuation 53 dB Output Power 18.3 dBm Superframes is 9294612 with RS Correctable Errors at 10424 and RS Uncorrectable Errors at 3739. HEC Errors is 4108 and Total Cells is 1060288 with Data Cells at 6967917 and Bit Errors at 0.
Upstream: 414kbps. SNR Margin is 13.3 dB Attenuation 30.6 dB Output Power 11.3 dBm Superframes is 9312559 with RS Correctable Errors at 928 and RS Uncorrectable Errors at 29. HEC Errors is 0 and Total Cells is 937033 with Data Cells at 2903113 and Bit Errors at 0.
The Lan Wireless Stats Received is reporting Bytes 100128732 Packets 1020654 Errors 120626 and Drops 120644. Transmitted Bytes 326530246 Packets 1167855 Errors 903 Drops 903. I think the the wireless errors work out at 1%?
Wan Stats VPI/VCI 0/38 Protocol PPPoA Received: Packets 750042 Errors 0 Drops 0. Transmitted: Bytes 326828328 Packets 244772 with Errors 0 and Drops 0.
I'm fairly sure its not an equipment issue at my end but until the speed drops below 1.5 Mbps BT will not treat this as a fault. ADSL speed is from my perspective undergoing a continuous degradation not related to the modem technology in use.