What those stats show is not the SNR, but the SNR margin. It must be one of the most common misunderstandings posted to boards.
The actual SNR on your line (which is not being reported in the stats) is what ultimately defines how fast your line can pass data. However, if there was any attempt to actually run at that rate, the line would constantly lose sync due to variations in the electromagnetic noise in the environment. So what operators do is set a margin of safety, which can absorb some changes in the noise environment without losing sync. Typically this is set to a minimum 6db, but if a line is unstable it might be set to 9db or even 12db. (And some ADSL operators would even use 3db on very stable lines). If you see a high margin on a line achieving maximum, or near maximum speeds, that's good as it means there's an even larger margin before sync drops and is a characteristic of a good quality, short line.
What you are seeing is that margin is being eroded, very probably due to more cross-talk as market penetration increases. If the margin drops towards 6db, you will start seeing a loss of sync speed. You've clearly got a lot to spare on upstream, with less on downstream. Don't expect to see the margin drop below 6db as Openreach do not (as far as I know) allow modems to negotiate anything less.
The good news is that the rate of erosion in performance seems to drop off so there's a fair chance that you won't get hit. Secondly, should vectoring be enabled, then expect to see those margins increase a lot.