Regardless of how anyone chooses to describe it is a loft conversion and a habitable space, even if BC accepted that it was not a habitable space (unlikely) there is still work that has been carried out that is subject to B Regs, however due to the length of time involved they are statute barred from enforcement
You can apply for retrospective approval (regularisation Certificate), however you must still satisfy BC it complies with the B Regs applicable at the time the work was commenced, this may involve opening up and carrying out remedial work.
Indemnity insurance is available but I know nothing about it or its implications, however I have personally been involved with 3 Reg cert apps where indemnity insurance had been previously purchased and claims had been denied, as a consequence Reg Cert was applied for. I understand that litigation was in progress and I believe my report of non compliant work was to be used as evidence, so be cautious of following this route. I will stress that other than dealing with these purely as a BCO I had no other involvement.
Oddly Sunderland is the only authority I either haven't worked for or submitted any B Reg apps to in the NE.