Sky run all the north Wales exchanges off a single feed ("chained" in their lingo).
The 151 and changed reverse DNS probably mean little, it's just new to me and happened at exactly the same time as getting a new IP over DHCP and the mentioned changes to performance.
Interestingly on a traceroute I see that data passes via 2.* and bb.sky.com routers anyway.
There are a few complaints in the Sky forums about YouTube performance (which makes a change from general congestion related issues). I thought they might have been making changes to deal with it.
For comparison, I had a second FTTC line installed last week, a BT one. Throughput is similar, but pings on that line are 6ms. Sky has never been lower than 20ms, and currently stuck at 30. Next time YouTube or iPlayer goes on the blink I'll be able to do 1:1 comparison checks over the two lines.