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The answer is not straightforward. There is no specific law that address light up dust caps. But there are regulations to do with vehicle lighting and accessories that can affect their use.
The Road Vehicles Lighting Regulations 1989, states that all lighting on a vehicle must be roadworthy. It must not cause undue distraction. It must not dazzle other road users.
So, if a light up dust caps or LED cap lights prove to be too bright , they would likely be deemed illegal.