Google's services have started working for me again, both wired and wirelessly. Not sure if they've fixed the issue and are rolling it out bit by bit but two others I was conversing with on another forum also report their services returning. One in Wrexham and the other in Staffordshire. I'm in Cheshire.
I now appear to be able to reach Google sites with both OpenDNS/Google DNS and BT DNS.
Videos which previously hadn't been working now do seem to work both with and without using the BT DNS, although the r4.sn-cu-ajtd.googlevideo.com address is still resolving to a BT IP address. However, the videos in question seem to be coming from a different address anyway now regardless of DNS provider, r4---sn-cu-ciml.googlevideo.com, which also resolves to a BT address, but does at least work.
Apologies for the inconvenience. We have worked with Google to fix this problem.