The worst case scenario would be that they stamp you in as a visitor and you have until XXXX date to present yourself to the Home Office with your ILR documentation (which would be fine as by that point you'll either have received your BRP or you could say, "check your records").
Likely case scenario is they make you sit to the side for 15 minutes while they verify your story, tell you that you aren't supposed to travel without your BRP in the future, and send you on your merry way.