Hi,
I have never had a problem with scales, in fact, I've always liked playing them. I believe that there are two parts to learning scales and arpeggios. One key issue is FINGERING, the other is KEY SIGNATURE. Knowing these two well is two-thirds of the battle, and since you are talking about grade 8, tempo is a final aspect, together with 'shaping' your scales.
With FINGERING if you look through the scales book you begin to notice that various scales share similar, if not the same fingering, e.g. C major/minor (melodic and harmonic) and E major/minor (melodic and harmonic) - I can't remember the grade 8 scales off the top of my head.
With KEY SIGNATURE, you just have to learn them - there is no other way off putting it. One way of doing so, is to initially learn them by starting with C major, then going through each major scale each time adding a sharp to your key signature, so next would be G major, F sharp, then D major, F sharp and C sharp etc. etc. Similarly, do this with major keys that have flats in them. Then repeat but looking at minor keys. This way of thought covers more than is required for grade 8, and is actually a grade 6 scales requirement. HOWEVER, this method will prove helpful in your sight-reading.
Hope this helps!