I would have expected the relevant Regulation to be here but I can't find it:
http://www.legislation.gov.uk/uksi/1986/1078/pdfs/uksi_19861078_en.pdf
However every motor vehicle on the roads needs to be approved for use by the DVSA. If you build your own car from scratch, for example, you can't use it on a public highway until such time as it has been examined at a DVSA testing station and granted 'approval'. Since it would obviously be impractical for the DVSA to inspect every single vehicle though, manufacturers seek 'type approval' for each of their models. Any later modification which, for example, changes the overall width of the vehicle (such as having tyres that protrude beyond the edge of the vehicle's body) automatically invalidates the 'type approval' and the vehicle can no longer be used on public roads unless it's submitted to DVSA and granted individual approval for its use.