Its not a grey area... they are illegal to retro-fit into halogen headlights.Their replacement headlights. Instead of using normal bulbs they use bulbs and a ballast to up the power.
You tend to see them in newer car like BMW's, they create a wall of light.
The legality is a grey area however I've had mine on two seperate cars now and I live in the inner city.
Police never said anything.