UCAS choices

I'm predicted ABB but might miss these grades . I have an offer from Bristol and Sussex for ABB and then Brighton CDD and Manchester and Oxf Brookes at BCC. I want to accept Bristol as the firm choice and then Sussex as back up as from what I can tell Sussex will take you with a slipped grade. Any views if this is a good idea. Also if I reject the Brighton offer ( which is well below the usual) and reapply does any one have experience whether or not they will repeat the very low offer