I'm.. not sure. Ian Allan publications' 1960's tome British Railway Signalling doesn't mention any signage denoting a change between colour light and semaphore signalled areas, but it does show a diagram of a special signal that had both an upper quadrant semaphore arm and a three-aspect colour light head on the same post, that was noted as "semaphore stop signal and colour light distant" and (used at) "at boundary of semaphore and colour light areas." It seems this particular signal specifically is a home or absolute signal for the semaphore arm (it's a LMS two aspect upper quadrant) and the light signal was specifically a distant (with two yellow indications and no green).
Apart from that, there is no mention about any particular signage used to denote the transition between colour light/power operated signalling to manual/semaphore signalling. Whether BR or before then, the Big Four, did, I have no idea. Hopefully someone more knowledagable on this subject would have a better idea of this I'm sorry.
I don't remember here in Australia any signage of semaphore to light signal areas on the main line - it seemed to me knowledge of what signals are used in what area is actually part of a driver's road experience training and so the lack of signage denoting that. But again someone more knowledgable on that subject would be able to clarify further.
