Lights

Since most dictionary definitions of "manufacture" seem to include some proviso about industrial/mass process and all my bike have been assembled from parts, does this mean I am exempt from this rule?

If you assemble a bike from parts, it is exempt from some regulations on construction, but regulations on its use still apply. Lights'n'reflectors regulations are construction & use regulations, so in some areas you have to use the equipment which would have been supplied as a result of the regulated construction, in others you have to use certain equipment even if it was not supplied with the vehicle, and in others you don't even have to use the equipment which was a mandatory fitment when you purchased the vehicle, e.g. front and spoke reflectors have to be fitted to new, complete, adult cycles sold for road use, but you don't have to leave them on even if you ride after dark.

You'd need to pore over your HMSO publications with a fine-tooth comb to come to a complete view over exactly which bicycles needed which nocturnal illumination.