It depends what sort of diploma you're doing.
Sags_3 is almost correct in his description of what prerequisites you'd need if you wish to take the performance diploma (although you don't actually need a distinction in your grade 8 to be eligible to take the exam)
Similarly if you wish to take the directing diploma, the prerequisite is grade 8 in a relevant instrument.
On the other hand, if you wish to take the teaching diploma, you do need grade 6 theory in addition to grade 8 in the instrument concerned.
Either way, your starting point has to be the diploma syllabus. Details are
here.Best of luck with whichever path you choose to take.