The UK government actually has the laws on Xenons wrong. The DfT site states that UK law regarding Xenons is based on EU directives. According to those EU directives a self levelling system is only required if your cars cut off moves by x% when the axels are placed under maximum specified load, ie people i the car or boot full. This means that if your car passed the displacement test with halogen lights then it will therefore also pass the same test with xenon bulbs in xenon designed projectors as the light pattern is the same as is the movement of the car under load. It then also states that washer systems are not mandatory UNLESS your car requires a levelling system. If these regulations were correctly followed by the DfT then all you would need is the correct projectors and bulbs, you could leave out the self levelling and washer system as they are not present on the halogen equiped cars, they have manual levelling.
I've sent these points along with copies of the EU article and regulations to the DfT and several tuning magazines but with no luck, it's a real shame that some one in a position to do so can't take this on as it would allow for a legal and relatively cheaper route to improved lighting on older cars.