Hi Michael,
The requirement is highly questionable. I have recent experience with CGI_XML_CT in UK among other countries. For intra-UK payments only IBAN is provided and bank is fine with that. No repairs or additional fees as far as I know.
In native CGI_XML_CT there is no such tag as BBAN. May be you can tweak FPAYHX-UBKNT to get it populated with BBAN . This field is source for tag <DbtrAcct><Id><Othr><Cd> . This tag could contain BBAN as per ISO 20022 External Code List - 3 codes: BBAN, CUID, UPIC.
But first I would ask your bank in which tag they expect BBAN and why simply IBAN is not sufficient for BACS (in CGI terms - NORM-NURG) payment in UK.
Best regards,
Anton