You will need to find someone to provide your UCAS reference. Your school might be willing to do that. If you a retaking but not studying in school (i.e. an independent exam candidate) unless you are very lucky it is unlikely the school will provide predicted grades. You cannot re-use the predictions you had previously (that will have been shown to be incorrect anyway if you are right about your results). If you engage a private tutor they might be willing to act as a reference and provide predicted grades. Otherwise you might need to apply without predictions and ask your referee to mention your personal targets in the reference.