Definatley register with Audax UK, then I'd try and contact the organiser ahead of the ride to check you can just turn up - presumably if your registration is being processed you could pay the 'on the day fee' (assuming this particular organiser will allow that) and then have the ride registered with the AUK website at a later date (when the organiser gets round to doing it)...
Spenceey - which Audax did you want to do?
Definatley register with Audax UK, then I'd try and contact the organiser ahead of the ride to check you can just turn up - presumably if your registration is being processed you could pay the 'on the day fee' (assuming this particular organiser will allow that) and then have the ride registered with the AUK website at a later date (when the organiser gets round to doing it)...