Managed to find this - it seems BII relates to having qualified as an Army farrier.
"Requests for Registration by British Army Farriers
If a qualified Army farrier wishes to be registered, he/she must provide an official certificate signed by
the Farrier Sergeant Major and the Commandant DAC stating the person concerned has passed the
relevant Army Trade Test. The Army BII or IMFC equates to the DipWCF and the Army BI or
AMFC to the AWCF. The fee is £250 but for serving personnel who apply within 6 weeks of their
Army Trade Test this is discounted to £125. "