Why the rush? There's a huge leap between Grade VIII and a diploma, and plenty of repertoire to explore in the meantime. If you want a definite target to aim for, is there a local music festival you could enter? If it has to be an exam, why not look at the Trinity Guildhall Performer's Certificate? It seems to bridge the gap between Grade VIII and a diploma. Could you put on a concert, maybe a fund-raiser with friends, for a local good cause?
Full details as to what a diploma involves are on the diplomas page of the main site, but in summary you have to give a 35 minute recital, a quick study (for DipABRSM this is five minutes to prepare, then perform, a piece of Grade VI standard), and a viva voce, which lasts for up to twelve minutes and is a discussion of your recital programme, any issues surrounding it, and based on the programme notes you have prepared.
Firstly, make time to discuss it with your teacher