Having seen several threads/posts on upgrading from Halogen to HID Xenon headlamps the link (below) details the legal requirements for the UK to carry out the conversion. Might be of use if you are thinking of going down this route.
http://www.dft.gov.uk/pgr/roadsafety/drs/hidheadlamps
http://www.dft.gov.uk/pgr/roadsafety/drs/hidheadlamps