You can take Trinity Guildhall exams up past G5 without having theory.. I think you only need it for G8 on Trinity Guildhall, so maybe use that board? It doesn't really make any difference.
However, I started the G1 book in about Y9, I got my G5 in Y12 (just in time!) but could have done it a lot quicker. It depends on how much you already know to be honest, but if you're up to G5 practical, you should know a fair bit already.