I would say that if you are showing in Mountain and Moorland it would count against you as they should be 'natural,
As for sweet itch, get some pure Neem oil. Mix 2 tablespoons Neem, ditto of shampoo add 1 L hot water and sponge that all over your horse. It helps keep all biting flies away and also sloths irritations.
Sponge it on so the coat is wet rather than spray it.
Neem solidifies so you may need to stand it in hot water or the sun to do solve it.
The downside to this is that it stinks! You do get use to the smell and it wears off fairly fast.