See: http://www.legislation.gov.uk/uksi/1986/1078/regulation/99/m... , https://www.legislation.gov.uk/ukpga/1972/20/section/196/ena...
https://www.racfoundation.org/motoring-faqs/mobility#a6 suggests it's about 35% of vehicles in large settlements.
See: http://www.legislation.gov.uk/uksi/1986/1078/regulation/99/m... , https://www.legislation.gov.uk/ukpga/1972/20/section/196/ena...