My school didn't give me any predicted grades except for the subject I was applying for (my chemistry teacher wrote my predicted grade only for chemistry in her reference), as there was no place on the UCAS form for this any more, so they assumed we didn't need any predicted grades.
Out of four of us applying to Oxbridge that year, we all got interviews (and offers), even though none of us had any predicted grades submitted. But I should think that, especially if your school is in the UK, you would be expected to have some, and I shouldn't imagine that they'd quite settle for a predicted grade lower than their usual minimum IB offer (ie 38 points with mostly 7s in your HL subjects).