Get your statement off by December of you're not doing medicine/vet or an Oxbridge candidate. It helps to have it go kind of early but you don't need to have everything ready to go on 6th September. Chill out, let your school help you and redraft it to get it really bloody good - because my statement is what got me an offer from my firm, not my predicted grades. Don't try and be a smart arse, or humour on the statement, it doesn't work. Just show off exactly what you can do.