'Could anybody please tell me in what year the requirement was introduced for intending FIs to possess either a CPL or a BCPL prior to taking the FI course?'
There has never been such a requirement!
ICAO Annex 1 has always specified CPL level knowledge as a pre-requisite to become a Flight Instructor. The UK did not apply this as it had an "equivalent" i.e. a Pre-entry examination, and a course of FI training. In approximately 1988, when the BCPL was introduced in line with the Annex 1 requirement for a 200 hour CPL, CPL theoretical knowledge was introduced in the UK as a requirement for all future FIs. The BCPL, CPL etc are only required for the purpose of "remuneration", an entirely different issue. On the introduction of the new theoretrical requirements; existing FIs operating on a PPL, were given a BCPL(Restricted) so that they could continue being remunerated as a grandfather right.