For Cardiff, looks like <8% of English applicants got an offer for 2023 entry - they are all likely to have had 9 x A* equivalents and/or been strongly contextual
6.5% Northern Irish applicants
5% scottish applicants
and 27% of welsh applicants
I would be very surprised if only 30% of Cardiff applicants score 27/27, given they give points for acchieved A levels as well as GCSEs
Every year, for the 10 or so years I have been doing this, people think they have a strong application for Cardiff and every year, multiple people with full academic points who tailored their PS to Cardiff are rejected, even with high UCAT. My guess is that the 3 extra points available to Welsh (and presumably contextual) applicants are just that - extra points. So people are actually scored out of 30, but the most a non-contextual can score is 27 and the cut off has been >27! But Cardiff will not confirm how they score people and past cut offs, so it remains impossible to recommend them unless there are really no other options - and you have other options
Of course, some people get in to Cardiff, but I am not sure any non-Welsh, non-contextual applicants do. They do have a very complex 300 point contextual system, so it is likely people get some points, even if they are not aware of it and this may explain those who are not typically WP who get an interview.