The BCPL became redundant when the hours required for a full CPL was reduced from 700 to 200 bringing the UK pretty much into line with the rest of the world. It was introduced when it became illegal to get paid for instructing on a PPL. At its introduction anyone instructing on a PPL prior to 1988 was granted a BCPL restricted to instructing only. To convert it to a full CPL you needed to do the CPL ground exams and flight test.
After the cut off date, which if I remember rightly was in 1988, anyone wanting to instruct had to do the CPL written exams and flight test, and that qualification could be converted to a full CPL with no further exams when the 700 hour requirement was met.