Leaflet INF104 is the thing to refer to:
https://assets.publishing.service.gov.u ... inf104.pdfThis latest version (published March 2020, I think) first of all says:
"Vehicles manufactured after 1 January 1973 must display number plates:
• made of a reflective material
• with a white background at the front of the vehicle and a yellow background at the back of the vehicle
• with black letters and numbers."
And then a bit later on says:
"Vehicles made before 1 January 1973 may display traditional ‘black and white’ number plates (for example, white, silver or grey characters on a black plate). From April 2020 vehicles manufactured before 1 January 1980 are also able to display traditional ‘black and white’ number plates. You must:
• have applied to DVLA
• be registered within the ‘historic vehicles’ tax class
The 40-year exemption date rolls forward automatically each year on 1 April. "