Some games released in the UK use the British Board of Film Classification (BBFC) for their rating system - Our Age Ratings | BBFC .
It is always possible to enter the correct rating in the app, but when submitting to core, a couple of the age ratings are not available - specifically U (Universal) and 12A. I don’t think that 12A has ever been used on a game (it is specific to allowing a child under the age of 12 to be permitted to watch a film in a cinema only if accompanied by an adult), but U is fairly common and should be available when selecting the rating for UK releases. For example BBFC | BBFC
Other rating organisations are represented, but not the BBFC.