Thanks for spotting the errors. It's actually 0.7%, not 0.5% of cohabiting/married couples being black and white. Asian and black would be 0.1%, not 0.003%. Are those less ridiculous now? If not, take it up with the Office of National Statistics.

https://www.ons.gov.uk/peoplepopulationandcommunity/birthsdeathsandmarriages/marriagecohabitationandcivilpartnerships/adhocs/12186ethnicitywithinmarriageorcohabitingmixedsexrelationshipuk2018

If unsure, you can cross-reference that with a 2018 report on ethnicities. You'll notice the proportions of single-race pairings is proportional with representation in the general population, maybe just a couple of percentage points lower to reflect the mixed-race couples. See https://www.ethnicity-facts-figures.service.gov.uk/uk-population-by-ethnicity/national-and-regional-populations/population-of-england-and-wales/latest#by-ethnicity