It might be possible to raise that predicted B to an A if you convince your teacher that an A is what you are going to acheive. If you are up to date on homework, can provide a vast amount of independent work as evidence of your dedication and are acheiving well in class you may be able to have a word with them, explain your situation and tell them that you are very confident that you can get an A. (It may also help if you are retaking a unit or two.)
Otherwise it depends on the university. I'm not familliar with Manchester but some universities will turn you away on predicted grades alone if they're hugely oversubscribed but others may invite you to interview which would give you a chance to explain to the university that you're aiming to meet their requirements. You'd be best to email the admissions team at the university who may be able to help you more.