Don´t know about UK regulations, but in Fin we have this kind of regulations for xenons:
-Cars registered for traffic 1.10.2000 must have: light washers, automatic height levelling and whole light unit for xenons (letters on unit: DC for low beam, DR for high beam, or DCR for both, halogens have only HCR marks).
-Cars registered 14.6.1995 and after: height adjuster (manual in cabin is ok), light units as above, light washers if xenons are installed afterwards (not stock).
-Cars registered before 14.6.1995: light units as above, light washers if xenons are installed afterwards (not stock).