I would just be honest and tell them that you are going BR. Do you have a date for your BR yet? If you have a date tell them - it might not make a difference or stop the phone calls but it's worth a try.
Tomorrow is a mystery, yesterday is history, today is the present, a gift to make the most of.
Just tell them that you are declaring BR and then don't bother answering their calls again until you have been declared BR. If they call again once you have been declared BR just give them the OR's details, case number, etc. and they will then deal directly with the OR.
Or just tell them you are seeking advice from the CAB/CCCS about your financial situation and some one will be in touch with them as soon as you have your interview, the OR :D