I was curious about this too so had a look. The brightness is regulated by The Road Vehicle Lighting Regulations 1989 and does specify a minimum wattage in some cases but not for modern cars (“A motor vehicle with four or more wheels first used on or after 1st April 1986: no requirement”). As far as I can tell there are no legal maximums.
https://www.legislation.gov.uk/uksi/1989/1796/schedule/4