NO...first u must have self leveling headlamps or suspension and head lamps constucted for the type of beam they have, but have a look at this link. Daniel Stern Lighting Consultancy and Supply
If u have 300 style lamps try the 80% upgrade bulbs they do work.
IF you are talking about replacing your existing bulbs with Xenon gas bulbs, then they are legal so long as you don't exceed the original power - 55/60w. They ae a worthwhile, cost-effective mod imo.
If you are talking about fitting Xenon gas headlights (with ballasts etc) that is a completely different thing and you need to have headlamp wash and self levelling to be legal. Incidently, even though Xenon gas headlights are a lot brighter than std, they are actually lower wattage, 35w from memory.