Clarkie 172 is right they arent illegal. But the quality of the cheaper ones are ****e, and may as well be illegal. I fitted some cheap ones which didn't last silver very long, after a week the silver tarnished to a black mess. Also on a bright day, it was difficult to see them flashing
(a few motorist let me know this via their car horn
). So stick to reconised brands such as phillips or Osram, they might be more exspensive but no doubt better quality.
So the moral of the story is "buy cheap buy twice"