In addition to https://status.aa.net.uk/1891 there is a UK wide problem with lines logging in. This is affecting other ISPs, and affecting a small number of lines. BT are already aware.
BT are saying this is now fixed. We saw affected lines come back online just after 10am. BT say about half of the UK 21CN WBC lines were affected, however, we only saw a few dozen lines affected.