UK readers may not care but North American regulations require that the length of such insulated conductors be not less than 6 Inches (150 mm), and I agree with that.
I've seen "professional" jobs where there is only enough length to form the wires to go in the terminals, slip the socket on, and "just" get a screwdriver in to tighten the terminal screws. And the ceiling roses had wires that wouldn't have stood cutting back and re-terminating.
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