LVPS are determined by individual airfield requirements.
In cap 168 it defines three stages of visibilities. They do not apply to all airfields, some stages can be skipped.
The basis of these levels depends on the equipment at the airfield, Cat 1, 2, or 3 ILS, the correct lighting to supplement all of these categories.
I cannot rememeber word for word but simply put the stages are
1 - The visibility condition when the controller cannot see the aircraft. Useage of SMR etc is allowed
2 - The visibility condition where pilots have limited visibility and visibility decreases the ability for a pilot to seperate themselves from other on the ground.
3 - Really bad vis but I Cant rememeber the criteria!!!
As an example, at Cardiff, we do not have any means other than mark 1 eye ball for seperating aircraft on the ground. Therefore, it is deemed that visibility condition 1 exists when you cannot see both ends of the runway (1500M) so that is when we commence LVPS. (Safeguarding commences before this time).
Heathrow, and many other uk airports have SMR to assist them in seperation on the ground, therefore, they have reduced the minimium criteria before it is deemed that Visibility 1 exists.
Hope that helps, although it is a little vague. Im off to read about snow now!!!
TIO