Hi OP,
Unfortunately there are many, many factors at play in relation to undergraduate intake each year. Of course, the department will consider all students who reach the grade requirements, and after that point, it all comes down to these things:
1. How popular is the course this year/how many students applied in general - the more that apply, the harder it will be.
2. How many students narrowly missed the grades and how man completely didn't get them (obviously your idea that you may get an A* BB instead of an AAB grade set is encouraging as it really isn't far off).
3. If you over-achieve on a grade and under-achieve on others, they will assess whether your A* was in a relevant subject i.e. History or Sociology.
4. Whether you interviewed - some courses interview and their offers are based on who interviewed really well.
5. Finally and most obviously, whether the course goes into clearing or not.
If it helps, I think you should not worry too much. If you get A*BB that is not too far off the grades you needed. I did a course that only allowed 55 students in, and I had to interview, and I ended up getting A*ABC when I needed AAB and still smashed the offer and acceptance.
Any other questions, please ask!