A warning signal is only provided when where a calling on signal is not a safe braking distance from platform. Train detected by delta track before signal and if moving at slow walking space signalman can clear signal, visual indication given in signal cabin. 1/ Proceed at caution 2/ Allow train to enter section ahead as far as next stop signal. 3/ Be prepared to stop at that signal as section ahead occupied.
Calling on Signal
Train has to stop before signalman can clear signal. Visual indication given in signal cabin that train is stopped. Delta track (11' 83/4") detects train. 1/ Proceed at caution once cleared 2/ Section being entered is occupied 3/ Be prepared to stop 10' short of train.
Seems like the warning signals on BR fell out of general use by 1970, apart from the one at Barry! I don't recall ever seeing one, I worked at Rugby and Kings Cross as a secondman, then Addiscombe and Waterloo as driver. I still can't recall seeing one on my travels!
From a brief (and I stress the word) glance at the modern edition of the Signalling Rule Book I can find no mention of them either, they're certainly on their way out if there are any left.
They were superseded by a 'Warning' class of route - basically a delayed yellow aspect. Remember that BR/NR had much stricter rules about signal spacing than LU, and only arbitary overlap distances.
On LT braking distances were more predictable due to fixed train lengths (LOL, pre-Westcode and anti-slip brakes); on BR train lengths and braking distances varied considerably, anything from a 80 ton unfitted goods train at 25 mph to an inter-city 125 or a suburban EMU! But yes. on BR the signal being cleared when the train was at or nearly at a stand was regarded as the warning signal where no separate aspect was provided. As I said earlier, all to do with overlaps and braking distances. This also accounts for the banner type distant signals that used to be provided on the Met main line.