Banking2006 is correct.... the auditors to date are only trained to assess new buildings, a follow-on course will have to be done to be qualified to assess existing buildings.
regarding your question...
The BER rating you will get done is not designed to be advisory. You may be able to engage the assessor to do a preliminary rating, then advise on energy saving issues, and then do a BER rating after these issues are resolved..... but generally the assessors are to be considered licensed agents of the government and not employees of a client....