How so?
One set of lights has BS markings and is legal, the other doesn't and is illegal.
Not actually the case...... lights can comply with the Road Vehicle Lighting Regulations 1989 (amended in 1994, 1996, 2001, twice in 2005 and again in 2009) and be legal for use, but not comply with any of the British Standards.
Even so it is possible to perfectly use a light that complies with neither RVLR or British Standards if it does comply with any other European Standard. B&M lights are for instance legal as they have a German K classification.
Equally these laws only apply to the primary lighting.
All my bikes have a set of "legal" lights, but I also run a pair of Dinottes on the back and a 720 Lumen LED on the front. All these are perfectly "legal" as they are a backup system..... and therefore do not need to comply as I fulfil the law with the small cheap LEDs
Most Police take the attitude that if a cycle is well lit and not dazzling other road users ten that is fine, especially as in my case the presence of a BS / K / or other marking on the USE or Dinotte lights is completely irrelevant.