"Quoting from the DfT document:-

The Commission proposes to introduce a definition for a roadworthiness test that components of the vehicle must comply with characteristics at the time of first registration. This may prevent most modifications to vehicles without further approval of the vehicle. (this will apply to many components and to all types of vehicle)"

http://www.the-ace.org.uk/armageddon/

Not that I pimp my Octy, but some here might...

http://www.briskoda.net/forums/topic/247696-the-eus-thoughts-on-modified-cars

Some more reading.