Several years ago my nephew and his wife (who was registered disabled) had a conversion to their kitchen including a wet room done by Manchester city council. Where did they fit the CU? In a bottom cupboard! My nephews wife could not get down to even see it. After me leaving a note to the...
Just a thought, Have the lights been converted to emergency lights? A block i look after have maintained emergency lights which over the years have been changed to LED's
Firstly, what is the armoured connected to in the house? Secondly you do not need a junction box, you can run from the existing socket as you will in effect have a radial circuit. Thirdly you can run from the second socket to a switched fused connector to control the light(with a 3a fuse fitted)...
Does the neutral for the outside light come from the same JB? If so run a new live to the new switch as you suggest and connect the existing switch live to the other terminal.
If you have a MAIN BOND of 10mmsq in your meter cupboard connected to the water pipe within 600mm of the water pipe coming in to your property the extra bonding is pointless, especially if you have plastic or other non-conductive materials.
If the lamp holder is a bayonet type it does not really make much difference, but if a screw fitting then use a multimeter to check which one goes to the centre contact and that will be the live.