They don't give offers out based on UCAT scores per se BUT they also kind of do with how the admissions process...
(Most, not all) unis now rank applicants largely based on their UCAT score (+ also GCSEs, etc...) and then chronologically invite them in batches from highest to lowest scores, therefore the for first batch of interviewees there are more spots on the course available and so applicants are more likely to get a offer. However as more applicants accept offers there are less spots up for grabs so unis give out increasing less offers per batch. So if you are in the lowest UCAT batch, there are the same amount of people competing for even less spots... Therefore for those in the batch with the highest UCAT scores it is easier to get an offer than those in the batch in the lowest UCAT scores.
That being said, those with high UCAT scores may also get multiple offers so may free up some spots for lower UCAT score holders...