According to someone who knows someone ... this is not a simple power failure. There has been a "failure" of core routers via the power supplies/backup power supplies to these routers. A targeted attack has not been ruled out. In some areas the network has up to six levels of resiliency, and these outer resilient layers have been targeted first.
This has been across multiple data centres, not all in the UK. In fact the primary UK data centre was the last to fall off the network. The underlying applications are mostly available, but inaccessible due to the network being down.