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Pretty sure it does relate to bikes:
http://www.legislation.gov.uk/uksi/1989/1796/schedule/2/made
but weirdly not cars built before 1986 or with a top speed of less than 25mph and a few other exceptions.
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You are referring, as is the legislation, to lights fitted to a vehicle not to a person's head. The bike is required to have a light, front and rear, in any case. The fact that someone head happens to be over 1.5 metres from the ground is beside the point and nothing to do with this legislation. If the bike was a penny farthing the light could potentially be higher than that due to the construction of the machine.
Light heights of a maximum of 1500mm, in the UK, relates to motor vehicles and definitely not bicycles.