As has already been mentioned, distance to the cab or exchange is of little relevance. You are talking just meters and its kilometers that add on the latency.. so it depends where in the UK you are located. Im up in the NW and until fairly recently then 16ms was considered the average time to get from the NW to London.
BTwholesale have recently been rejigging their network and bringing in MSE bRAS which shaves an extra couple of seconds off that time.
The other thing to mention is that we have been noticing that customers of BTretail do seem to experience slightly higher latency than some of the other UK ISPs
From your stats you obviously have interleaving and error correction (INP) applied so that will account for additional delay. I would not class 17ms bad for a line which has interleaving applied.
I would not let that line run at SNR margins of 3.8 and 1.8 and suggest you perform a resync to bring your SNRm back to the default 6dB. Its the low SNRm which will be causing errors to accrue, which is what will make DLM apply interleaving. When errors are generated then this is when you can sometimes see latency very briefly start to lag and may account for the higher latency you say you sometimes see.
I know the line is currently running at 20Mbps upstream and a resync could give you a lower speed, but if you dont do it, then DLM will do it for you and add error protection.
I'd also be monitoring/graphing your SNRm to see how much it varies over the course of the day. Based on your downstream atten then you should easily be able to manage 20Mbps and if you cant, then its suggestive of faulty internal wiring or a line fault.
If you do a quite line test can you hear any noise?
