I have a new adult pupil starting who wants to learn theory! Is grade 5 theory necessary, well I never took it. In my day there was an O level music opt out. With a degree in music, I certainly don't feel I've missed out.
Strangely enough I like theory, but then I did major in musicology, but the hoops that the ABRSM put people through though academically interesting are not exactly necessary. If your teacher knows their stuff and can talk about periods of music, how to ornament effectively for each period of music, who wrote what when, you'll learn far more that way than by jumping through the theory hoops.
Better get a teacher who has had a rounded musical education than rely on ABRSM theory exams to get you through.