maybe they were so adamant because of your predictions, and think you might relax a bit if you know getting AAB is an option
i just know that my friend who was predicted AAA, phoned them up and they said theyd'd take him if he got AAB
if i were you i'd go and sit the lnat and do whatever i could to get AAA, birmingham is heads above the other universities you have applied for and will serve you much better in later life, as a birmingham degree is very respected
also if you were to take a gap year, in order to apply to "better" universities, and you only had ABB then i'm afraid you wouldn't be able to apply to "better" universities as most of them ask for AAA, and the odd one ask for AAB
if i were you i'd sit the lnat, firm birmingham, and if you don't get in you can drop out of the cycle then and reapply if that's what you really want to do