I would imagine these requests would be dealt with by each creditors Data Controller -- details of which can be gleaned from the Information Commissioner's Website (ICO)
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014
Whilst every organisation has to provide a central address for their Data Protection Officer and for where Subject Access Requests can be made to, the practical reality is you can make the request at your local branch, and they have the responsibility to act on it and make sure it gets to the right place.