I am only speaking in general terms. C to CE training has been accepted since the start of dcpc - 2009 for trucks. In the early days, it was also accepted for C1 to C but it got stopped. Doing a course directly to CE would not fit the way the cpc course is written - and so it couldn’t be utilised. Maybe someone will take the time to write a B to CE course. It wont be me as the demand for periodic cpc at licence acquisition stage has dropped substantially as the grandfather rights drivers are becoming less common.
I cannot explain the decisions made by JAUPT and DVSA; some make perfect sense whilst others simply baffle me.
Hope this helps a bit, Pete S 