My ceiling light won't turn off, how do I wire it up correctly? (UK)
It seems apparent from your second-light picture that the light carries both switched-hot for the lamp (on yellow) and onward always-hot power for onward points (on red-hot and blue-neutral).  So that tells us you are correct about cable A.  

Between cable B and C, one of them is more likely than not a switch loop.  I note B is a twin-and-earth using red+red conductors.  Use of specially color coded cable for switch loops is a British thing. That notable lack of color coding makes it only useful in switch loops, so that's cable B.  You can double-check by opening up your switch and checking the colors there. 

That leaves cable C.  Since the cable to the other lamp is accounted for, cable C can only be onward power to other points of use.  

When I say "onward power" in both boxes, one or both of those could be going to supply. I don't really care which one is supply and which one is onward to loads. It makes no difference here; the point is those cables are intended to have always-on power. 




So all neutrals should be joined together, including the neutral wire from this here lamp. 
All always-hots should be joined together, and if B is a switch loop, one of the two wires in B must be always-hot. - Then all switched-hots are joined together, in this case it'll be 3 wires: 1 wire from the switch loop, one to the other lamp, and the hot from this lamp.