Uni email and UCAS confusion

Hi, I applied to BU for Software Engineering and was very indecisive so I was declined after not making a choice in time. I got an email from BU telling me I still have time to make it my firm choice and need to submit a short form before 24th july in order to do so. I submitted the form and it said I need to ring up UCAS and tell them to change my reply, so I did. The lady told me that she cant change the status and would have to go through clearing, which confused me. The email clearly stated I still have time, so why was she telling me I have to go through clearing? Im just as confused as she probably was. Any help?

Keep calling and take the name of the people you speak to. As you have an email, ask to be put in contact with the person whose name is on it when you speak to the university and explain the situation.

It isn't the uni that I need to speak to, it's UCAS. The email BU sent told me to fill the form in and once that is done ring UCAS to tell them that I want to change my reply. Although, BU did say that they are going to get in contact with UCAS also.