With a ping that low - I'd say far more likely to be down to the routing - either peering or even BTW have done something on their part of the network.
I notice that my latency changes on occasion which has nothing to do with the PN network, but more to do with the stuff thats normally hidden by L2TP on the BTw backhaul.
BTw quite often change the routing on the ATM cloud. 8ms is pretty damn hot for an IPStream connection, so at a guess you must be down south and on a london RAS.
Ive been reRASed several times by BTw over the years and it does make a difference to your latency - Dont think that BTw will always route you via your nearest RAS either because they dont.:/
My current latency to the RAS is around 8ms, then an additional 6ms down to london.
Because of l2tp tunnelling its normally all hidden - but its still there..
the following is an example of the stuff we as end users dont normally see on the BTw part.
1 1 ms 1 ms 1 ms 192.168.7.100
2 8 ms 9 ms 8 ms 217.47.204.58
3 8 ms 8 ms 8 ms 217.47.251.161
4 8 ms 8 ms 8 ms 217.41.173.13
5 8 ms 9 ms 8 ms 212.140.206.74
6 11 ms 8 ms 7 ms 212.140.206.46
7 9 ms 7 ms 8 ms 217.47.158.82
8 7 ms 8 ms 8 ms core1-pos1-3.manchester.ukcore.bt.net [194.72.2.33]
9 16 ms 14 ms 14 ms core1-pos0-8-0-7.ilford.ukcore.bt.net [194.74.65.113]
10 12 ms 13 ms 13 ms core1-pos9-1.telehouse.ukcore.bt.net [194.74.65.114]
11 14 ms 13 ms 14 ms 194.74.65.6
On occasions Ive seen latency as high as 30ms just on the BT backhaul (non-interleaved).. but since my last reRAS its been pretty good.