Can I legally use front facing blue light in the UK?
Regulation 16 of the Road Vehicles Lighting Regulations (1989) states:


  Restrictions on fitting blue warning beacons, special warning lamps and similar devices
  
  
  No vehicle, other than an emergency vehicle, shall be fitted with–
  
  (a) a blue warning beacon or special warning lamp, or
  
  (b) a device which resembles a blue warning beacon or a special warning lamp, whether the same is in working order or not.
  


The Regulations have been updated multiple times since 1989, but Regulation&nbsp;16 has not changed.

A warning beacon is defined by the Regulation to be "A lamp that is capable of emitting a flashing or rotating beam of light throughout 360° in the horizontal plane" and a special warning lamp is "A lamp, fitted to the front or rear of a vehicle, capable of emitting a blue flashing light and not any other kind of light".  A blue flashing lamp would surely "resemble... a special warning lamp"; it's unclear whether a solid blue lamp does, but please just don't. Blue lights on emergency vehicles are useful mostly because blue uniquely means "emergency vehicle." People who see a blue light will be distracted trying to figure out if it is an emergency vehicle.

Stick to white lights on the front and red ones on the back. Note that the human eye is not very sensitive to blue (only about 20% as sensitive as to red and green), so blue is a poor choice anyway. See also Should unconventional colors be avoided for lighting?